WHAT IS THE EVIDENCE FOR THE EXISTENCE OF HIV?
By Valendar Turner
Department of Emergency Medicine, Royal Perth
Hospital, Perth, Western Australia
The real purpose of scientific method is to make sure Nature hasn't
misled you into thinking something you don't actually know... One logical
slip and an entire scientific edifice comes tumbling down. One false deduction
about the machine and you can get hung up indefinitely.
Robert Pirsig, Zen and the Art of Motorcycle
Does the currently available evidence prove beyond reasonable doubt
that a unique, exogenously acquired retrovirus has been isolated from the
tissues of AIDS patients? Perhaps. Perhaps not. This is what I invite you
to judge. And in case you are inclined to be assaulted by the opinions
of overwhelming majorities, you may take comfort from a most venerated,
international scientist who said, "In Science the authority embodied
in the opinion of thousands is not worth a spark of reason in one man."
I shall reward you with his identity at the end of this talk.
A virus is two things. Number one: It's a microscopic particle of certain
size and form. Number two, such particles generate identical progeny by
parasitising chemical constituents and energy from a living cell. This
is what is actually meant by the term infectious. It is this attribute
which justifies a particle being called a virus. This is the property which
prevents our calling every particle we see a virus. By definition, a retroviral
particle is spherical in shape and has a diameter of 100-120 Nm. On the
outside is a shell studded with outwardly projecting knobs, knobs obligatory
to latch on to and infect new cells. On the inside there is a core containing
RNA as well as some proteins, one of which is an enzyme called reverse
transcriptase. The latter gives retroviruses their name and its function
is to catalyse the transcription of viral RNA into DNA, that is, to copy
information contained in RNA in a direction opposite the customary direction,
DNA to RNA. According to virologists, it is the DNA copy of the RNA blueprint,
not the original RNA, which hibernates inside the cell nucleus awaiting
an opportune time to orchestrate the production of new viruses.
To analyse their constituents and to prove they are truly viruses, retroviral-like
particles must first be purified. This is done by a process called density
gradient ultracentrifugation, something that may sound complicated but
which isn't. A test tube containing a solution of sucrose, ordinary table
sugar, is prepared light at the top, but gradually becoming heavier towards
the bottom. A drop of fluid from a cell culture is gently placed on top
and the test-tube is centrifuged for several hours at extremely high speeds.
This generates forces many thousands of times that of gravity and any tiny
particles present are gradually forced through the sugar solution until
they reach a point where their buoyancy prevents them penetrating further.
For retroviral particles, this occurs where the density reaches 1.16 gm/ml,
the point where the particles concentrate or, to use virological jargon,
band. The 1.16 band can then be selectively extracted and photographed
with an electron microscope. So, to prove the existence of a retrovirus
one is obliged to:
1. Culture putatively infected cells.
2. Purify a sample in a sucrose density gradient.
3. Photograph the 1.16 band proving there are particles of the right
size and form, and there is no other material.
4. Extract and analyse the constituents of the particles and prove they
contain reverse transcriptase by showing they can make DNA from RNA.
5. Culture purified particles with virgin cells demonstrating that a
new set of particles appears with the same morphology and constituents
as the originals.
Now I am going to discuss some of the data from four papers published
in Science in May 1984 by Dr. Robert Gallo and his colleagues from
the US National Cancer Institute. These papers do not describe the original
discovery of what the overwhelming majority regard as HIV, that distinction
falls a year earlier to Professor Luc Montagnier and his colleagues from
the Pasteur Institute from where, it is important to say, samples were
sent to the Gallo laboratory and which later caused Gallo and his colleagues,
as well as the US government, quite a number of problems. Neither are the
Gallo papers the last word on HIV isolation but there is no doubt they
are most important because it was they that led to the famous Washington
press conference of April the 23rd 1984, two weeks prior to publication,
at which an anxious, waiting world was told that the cause of AIDS had
been identified. In fact, as one scrutinises the vast AIDS literature,
it is fair to say that of all the papers published on HIV isolation, including
the very latest, the Gallo papers are the most rigorous by far. The problem
is, are they rigorous enough?
The first paper begins with cultures made of T-lymphocyte cells from
AIDS patients. These cells were chosen because, included amongst their
numbers, are the putatively infected cells, a subgroup known as T4 lymphocytes.
It is these that are often diminished in AIDS, the hypothesis being that
the yet to be discovered retrovirus was infecting and killing them. After
an unspecified time, concentrated fluids from these T-cell cultures were
subcultured with cells of a stock, leukaemic T-cell line known as HT. In
these secondary cultures the Gallo team reported particles in electron
microscopic examination of gross, unrefined culture fluids and measured
reverse transcriptase activity in both these and banded specimens but without
evidence that retroviral- like particles or indeed any particles were present
at 1.16 gm/ml. They also reported reactions were seen between culture proteins
and some antibodies present in human and animal sera. From these data,
the Gallo team claimed to have isolated a new retrovirus, HIV, as well
as inducing it to grow in the HT cell line in large enough quantities for
use in analysis and diagnosis. In a subsequent third paper, from banded
culture fluids obtained from a disrupted HT cell clone, two proteins, and
for no other reason than they reacted with antibodies present in human
AIDS sera, were deemed to be the HIV proteins. Subsequent papers, published
after the Gallo four, using the same logic, increased the number of such
proteins to about ten.
Reading these data it is obvious that Gallo and his colleagues had abandoned
the traditional method of retrovirus isolation. This is enigmatic when
one realises that, in 1976, Gallo himself had stressed that the detection
of particles and reverse transcriptase, even reverse transcriptase inside
particles, are not proof of the existence of retrovirus because, no matter
how remarkably such particles may resemble retrovirus, many such particles
are not viruses because they totally lack the ability to replicate (Gallo
et al., 1976). You must appreciate the magnitude of the particle problem.
Cell cultures contain many and many kinds of particles, some viral-like
and some not. The viral-like include retroviral-like. In the 1970s, retroviral-like
particles were frequently observed in human leukaemia tissues (Gallo et
al., 1976), cultures of embryonic tissues, and "in the majority, if
not all, human placentas." (Panem, 1979) One genus of retroviral-like
particles, the type-C particle and the one into which Gallo classified
his newly discovered retrovirus HIV, is found in "fish, snakes, worms,
pheasant, quail, partridge, turkey, tree mouse and agouti" (Grafe,
1991) as well as in "tapeworms, insects...and mammals." (Frank,
1987) This being the case, there seems to be no way of avoiding the rules
developed over the decades of research into animal retroviruses, rules
that enabled a scientists to sort out this clutter. And there are two more
complicating factors. The first is that reverse transcription is not only
a property of retroviruses. Normal cells contain enzymes which reverse
transcribe RNA and so does hepatitis B virus, a virus that infects T-cells
as well as liver cells and is present in a considerable number of AIDS
patients. The second is the choice of the HT cell line. It was long known
that leukaemic cells theselves can reverse transcribe and, strange as it
may seem, although Dr. Gallo was about to look for reverse transcription
as a sign of a new retrovirus, the HT cell line originated from a patient
who, according to Dr. Gallo, had a disease caused by a retrovirus he discovered
called HTLV-I. In fact, in 1983, Gallo reported that the HT parental cell
line contained HTLV-I genetic sequences. On this basis alone one would
expect to find evidence of reverse transcription in the HT cell line. Given
all these data, one would imagine it was impossible for the Gallo team
to abandon the need to follow the traditional method and isolate and characterise
infectious particles, but abandon it they did. By what reasoning then did
the Gallo team claim to have proven the existence of a new retrovirus from
For their 1984 papers, they reiterated the limitations of particles
and reverse transcriptase and made three assumptions which, taken together,
constitute a precept known as specific reactivity. (Gallo et al., 1986)
The first assumption was that AIDS patients are infected with a replicating
retroviral particle, a virus which could be grown in cell cultures to yield
unique, virus-specific proteins. Second, being foreign, the virus would
stimulate the production of a number of distinctive antibodies directed
against the viral proteins. Third, the proteins and the antibodies react
specifically, that is, only with each other and with no other agent. Let
us take a very careful look at this paradigm. First, when the Gallo team
began their experiments, the existence of specific viral proteins as constituents
of a replicating viral particle which could infect humans was entirely
hypothesis, not fact. Second, antibodies and proteins are not monogamous,
even the purest of each take on other partners. Third, even if they were
monogamous, we know that AIDS patients contain antibodies to many different
agents, many with which they are infected, for example hepatitis B and
cytomegalic inclusion viruses, mycoplasma, fungi and mycobacteria. Unless
Gallo further hypothesised that all these agents or parts of them, or their
respective antibodies, disappear from cultures or sera, when blood from
an AIDS patient is mixed with cell cultures of the same or another AIDS
patient, how can anyone tell what is reacting with what, let alone define
precisely where each of the reactants originated? As far as the reactions
are concerned, it's no different from mixing up milk from six species of
animals, adding a mixture of a dozen different acids and claiming to know
which acid is curdling which milk. So, although the term specific HIV proteins
conjures up visions of proteins being extracted from retroviral-like particles
proven to be a unique virus, this is not how it was done. It was done by
breaking up cells of the HT cell line, not a virus particle, and observing
unknown proteins reacting with unknown antibodies. From these data both
the proteins and the antibodies were deemed viral, and not just any virus,
but HIV. That's all. Logic or magic? And as an aside, similar to the proteins,
the origin of what is called the HIV genome, the HIV RNA, is also based
on circumstance, not on purification and dissection of particles proven
to be infectious. The Gallo team may have claimed isolation of a new retrovirus
but what they actually did was weave a nexus between reverse transcriptase,
particles, and certain proteins under the dubious imprimatur of specific
reactivity. Is this virus isolation? Is this even virus detection?
There are also a number of unsolved mysteries in the Gallo papers.
Mystery number one:
Reading the first paper one gets the impression that the HT cell line
was cultured with individual AIDS patient cultures. However, the National
Institutes of Health enquiry instigated after allegations of misappropriation
of the French specimens found that the HT cell line was cultured with concentrated
fluids pooled initially from individual cultures of three patients and
ultimately from the individual cultures of ten patients. (Maddox, 1992)
In evidence given to the enquiry, the reason given was because none of
the individual cultures "was producing high concentrations of reverse
transcriptase." That means not enough to convince the Gallo team of
scientists or anybody else there actually was a virus in any of the individual
specimens in the first place. The fact that pooled specimens produced reverse
transcription is not proof of a retrovirus. The conditions may have merely
changed in favour of the action of one of the cellular enzymes that performs
the same trick. Or it could have been due to the HT cell line, unaided
or at the behest of its HTLV-I retrovirus. The Gallo investigation found
the pooling of specimens "of dubious scientific rigor." One scientist
described the procedure as "really crazy." In essence, it is
no different from investigating an outbreak of pneumonia by having all
patients spit in separate pots and, when nothing turns up, getting them
all to spit in the same pot.
Mystery number two:
The method of specific reactivity required a source of antibodies to
the putative viral proteins. The logical place to obtain these was from
AIDS patients -- after all, that is what the hypothesis required. The antibodies
reported in the first paper appeared from two sources, a haemophiliac patient
known as E.T. who had pre-AIDS and rabbits. Yes, rabbits. What precisely
constituted E.T's pre-AIDS we are not told, but pre-AIDS is often generalised
enlargement of the lymph nodes, a condition not invariably followed by
AIDS and which is not AIDS. Thus, according to the paradigm of specific
reactivity, we cannot be sure that E.T. actually had the right kind of
antibodies. Rabbits do not develop AIDS and if specific antibodies to a
retrovirus were to exist, they could only be produced by immunising rabbits
with pure virus or, as the first Gallo group paper reported, from rabbits
infected repeatedly with disrupted HIV. I hope you are beginning to see
the problem. To make antibodies just to HIV, one has to inject rabbits
with pure HIV. Pure virus means isolated virus and if rabbits were injected
with pure virus, why should it be necessary to produce antibodies to define
the isolation of virus that had already been isolated?
Mystery number three:
In the second paper, the Gallo team attempted what they called HIV isolation
from 72 AIDS patients. Again, they cultured cells and detected particles
and reverse transcriptase in unrefined culture fluids, and observed some
protein/antibody reactions, but also added a fourth category, transmission,
by which was meant finding particles or reverse transcriptase in bone marrow
and other cells cultured with fluids, but not purified banded, photographed
fluids, from one of the 72 starting cultures. What is enigmatic about the
second paper is that HIV isolation was defined merely as detecting at least
two of any of these four phenomena. The same criticism applies as in the
first paper. Nothing was isolated and detection of unspecific phenomena
is not surrogate isolation of a retrovirus. Even it were, this peculiar
definition leads to some rather bizarre possibilities, for example, instances
of virus isolation without the need to see particles or measure reverse
transcriptase, for a retrovirus about as convincing as trying to sell a
car without a body and an engine. Even so, loose as these criteria were,
isolation was successful in only 26 of the 72 patients, that is, in only
36%. And, in case you think things have improved, there is a recent, international
cooperative study reported by the World Health Organisation. In this study,
by HIV isolation was meant detection of a single protein, p24, in culture
fluids using a single antibody. Not only is p24 not specific for HIV (Agbalika
et al., 1992; Mortimer et al., 1992), but from 224 HIV positive individuals,
the success rate was a mere 37%, not significantly better than Gallo's
figures a decade earlier. (WHO, 1994)
Even if the Gallo team had proved the existence of a new retrovirus,
on what basis did they claim it was the cause of AIDS? Even if virus had
been isolated from all patients and all patients had antibodies, which
they didn't because in the fourth paper, the data showed only 88% of AIDS
patients had antibodies (and on a single ELISA test which no one now regards
as specific), is this sufficient proof that HIV causes AIDS? If the bank
manager and his constant, faithful offsider are present at the bank robbery,
is this proof that the manager robbed the bank? The Gallo papers provide
no evidence whatsoever that HIV kills T4-cells or that low numbers of T4-cells
is necessary and sufficient for the development of the AIDS infections
and cancers and, I might add, there is still no such evidence. (Papadopulos-Eleopulos
et al., 1994)
Let me finish by summarising the problem. The method of retrovirus isolation
presented at the beginning flows logically from the definition of a virus.
It is model of intelligibility, it is the only method, and was used for
decades of research into animal retroviruses. (Sinoussi et al., 1973; Toplin,
1973) The problem is that, to date, nobody in the world has reported use
of this method in AIDS patients. Without it, for example, how can one resolve
the dilemma imposed by the numerous particles of stunning morphological
variability present in cell cultures of AIDS patients? Even so, although
some particles are the right diameter, there are no particles with the
right diameter AND the projecting knobs, both integral to the definition
of a retroviral particle and the latter essential to infect new cells.
(Gelderblom et al., 1988; Layne et al., 1992; Levy, 1996) Yet, as I speak,
there is still not even one published electron micrograph from a density
gradient to tell us which, if any constituents of this zoo of particles,
presents itself to be proved an infectious retrovirus. Perhaps, if someone
were to look, there might not be any. Reading the literature, it is obvious
that scientists everywhere have abandoned the traditional method of isolation
and, under the assumed aegis of specific reactivity, claimed that two unknowns,
antibodies and proteins, interact in specific pairs simultaneously betraying
each other's genesis from a virus. In other words, what is the guts of
what is called HIV isolation is actually no more than a chemical reaction,
an antibody test, and from an antibody test, one cannot claim proof of
isolation of anything. If an antibody test is isolation of a virus then
the pregnancy test, which uses an antibody to detect the placental hormone
beta HCG, must be regarded as placental isolation. Of course, there may
be instances of specific reactivity involving viral proteins and antibodies,
but the only way to prove this is to compare reactions in the test-tube
with the virus of interest. Nature would then reveal specific reactivity
by the fact that reactions, the virological equivalent of curdling milk,
show up only when there is virus and never if there is no virus. This is
crux of the matter and where the evidence for the existence of HIV begins
to fall apart. To prove specific reactivity, one must first isolate the
virus for use as a gold standard for comparison. One cannot adopt specific
reactivity as a premise to prove the existence of a virus if one must first
isolate the virus to prove the premise upon which isolation is contingent.
Try as you will but the cart does not go before the horse and the Gallo
argument is reductio ad absurdum.
This leaves us in a perilous quandary. What are these unknown antibodies
to unknown proteins which we call being HIV positive? They could represent
a virus, but that remains to be proven by isolating a retroviral-like particle
and proving it is a retrovirus. It is certainly not cogent to argue that
the conjunction of a number of unspecific phenomena makes one possibility
a definite outcome any more than claiming that ten men, all dressed in
white, hitting a ball around a paddock, must be playing cricket. They might
just as well be Ku Klux Klaners playing baseball. If not a virus, then
what? If someone tests positive, is this proof that a virus has been transmitted?
Or is it something altogether different? Whatever these reactions mean,
they do seem to be a marker for AIDS in the high risk groups, but are they
just as significant in those at low or at no risk? Does just knowing you're
HIV positive affect your health? Does your doctor knowing you're positive
lead to treatments for a virus you may not have? Could such treatments
themselves cause harm? We now know that antibodies to the germs that cause
the diseases present in 90% of AIDS patients also react with the so called
HIV proteins. (Muller et al., 1991; Kashala et al., 1994) Are we being
fooled by antibodies that have nothing to do with a retrovirus? Are we
seeing curdle from a different milk? Why, in one study, did 10% of 1300
individuals at low risk for AIDS including blood donors have antibodies
to a sufficient number of HIV proteins to deem them HIV infected by the
most stringent United States criteria? (Lundberg, 1988) Why do 30% of individuals
transfused with HIV negative blood develop antibodies to the same p24 protein
nearly every HIV researcher uses to "isolate" HIV? (Genesca et
al., 1989) Why do 50% of dogs have antibodies to one or more of these same
proteins? (Strandstrom et al., 1990) How come healthy, non-HIV-infected
mice injected with blood from similar mice, or mice injected with extracts
of a common human bowel bacterium, develop some of the same antibodies?
Why does transfusion of one's own, irradiated blood produce the same antibodies?
(Kozhemiakin & Bondarenko, 1992) If these data do not mean that HIV
antibodies are non-specific, then there must be some completely unknown
as well as very peculiar ways for men, dogs, and mice to acquire HIV infection.
On the other hand, if some humans, injected with their own or someone else's
blood, or mice injected with foreign cells and foreign proteins develop
"HIV antibodies" but are not infected with HIV, why should gay
men, IV drug users, and haemophiliacs, who are all exposed to foreign cells
and/or foreign proteins, not also develop "HIV antibodies" and
not be infected with HIV? Is it possible that we been misled by non-retroviral
phenomena altogether? This would not be the first time. Over the mid- to
late-1970s, Gallo and his colleagues claimed to have isolated the first
human retrovirus, HL23V, from patients with various types of leukaemia
and their evidence included a picture from a density gradient. (Gallagher
& Gallo, 1975; Gallo et al., 1976) Soon enough antibodies to the HL23V
proteins were found to be widespread, even amongst normal people and there
was great excitement that a cause of leukaemic was at last in the offing.
However, two groups of researchers then found that the antibodies were
in reality directed against a wide range of naturally occurring substances,
thus destroying that particular notion of specific reactivity. (Barbacid
et al., 1980; Snyder & Fleissner, 1980) Overnight, HL23V vanished from
the scientific literature, so much so that Gallo now never mentions it.
Does a similar fate await HIV? Neville Hodgkinson (Hodgkinson, 1996), the
former science and medical correspondent for the London Sunday Times,
has suggested that HIV is the greatest scientific blunder of the twentieth
century. If so, there are alternative theories and therapies for AIDS we
would do well to consider.
Now, are you ready for that scientist? His name is Galileo Galilei,
a man no stranger to heresy. Perhaps we should heed his counsel and begin
to trust our own sparks. I say the sooner the better. *
Agbalika, F., Ferchal, F., Garnier, J. P., Eugene, M.,
Bedrossian, J. & Lagrange, P. H., 1992. False-positive HIV antigens
related to emergence of a 25-30kD proteins detected in organ recipients.
Barbacid, M., Bolognesi, D. & Aaronson, S. A., 1980.
Humans have antibodies capable of recognizing oncoviral glycoproteins:
Demonstration that these antibodies are formed in response to cellular
modification of glycoproteins rather than as consequence of exposure to
virus. Proc. Natl. Acad. Sci. U S A 77:1617-1621.
Frank, H. 1987. Retroviridae. pp. 253-256, in Animal
Virus and Structure, edited by M. V. Nermut and A. C. Steven, Elsevier,
Gallagher, R. E. & Gallo, R. C., 1975. Type C RNA
Tumor Virus Isolated from Cultured Human Acute Myelogenous Leukemia Cells.
Gallo, R. C., Sarin, P. S., Kramarsky, B., Salahuddin,
Z., Markham, P. & Popovic, M., 1986. First isolation of HTLV-III. Nature
Gallo, R. C., Wong-Staal, F., Reitz, M., Gallagher, R.
E., Miller, N. & Gillepsie, D. H. 1976. Some evidence for infectious
type-C virus in humans. pp. 385-405, in Animal Virology, edited
by D. Balimore, A. S. Huang and C. F. Fox, Academic Press Inc., New York.
Gelderblom, H. R., Bozel, M., Hausmann, E. H. S., Winkel,
T., Pauli, G. & Koch, M. A., 1988. Fine Structure of Human Immunodeficiency
Virus (HIV), Immunolocalization of Structural Proteins and Virus-Cell Relation.
Micron Microscopica 19:41-60.
Genesca, J., Jett, B. W., Epstein, J. S. & Bloggs,
1989. What do Western Blot indeterminate patterns for Human Immunodeficiency
Virus mean in EIA-negative blood donors? Lancet ii:1023-1025.
Grafe, A., 1991. A History of Experimental Virology.
Hodgkinson, N., 1996. AIDS: The Failure of Contemporary
Science. Fourth Estate, London.
Kashala, O., Marlink, R., Ilunga, M., Diese, M., Gormus,
B., Xu, K., Mukeba, P., Kasongo, K. & Essex, M., 1994. Infection with
human immunodeficiency virus type 1 (HIV-1) and human T cell lymphotropic
viruses among leprosy patients and contacts: correlation between HIV-1
cross-reactivity and antibodies to lipoarabinomannan. J. Infect. Dis.
Kozhemiakin, L. A. & Bondarenko, I. G., 1992. Genomic
instability and AIDS. Biochimiia 57:1417-1426.
Layne, S. P., Merges, M. J., Dembo, M., Spouge, J. L.,
Conley, S. R., Moore, J. P., Raina, J. L., Renz, H., Gelderblom, H. R.
& Nara, P. L., 1992. Factors underlying spontaneous inactivation and
susceptibility to neutralization of human immunodeficiency virus. Virol.
Levy, J. A., 1996. Infection by human immunodeficiency
virus-CD4 is not enough. NEJM 335:1528-1530.
Lundberg, G. D., 1988. Serological Diagnosis of Human
Immunodeficiency Virus Infection by Western Blot Testing. JAMA 260:674-679.
Maddox, J., 1992. More on Gallo and Popovic. Nature
Mortimer, P., Codd, A., Connolly, J., Craske, J., Desselberger,
U., Eglin, R., Follett, E., Hawkins, J., Kurtz, J., Parry, J., Roome, A.,
Samuel, D., Skidmore, S. & Tedder, R., 1992. Towards error free HIV
diagnosis: notes on laboratory practice. Pub. Health Lab. Service
Micrbiol. Digest 9:61-64.
Muller, W. E. G., Bachmann, M., Weiler, B. E., Schroder,
H. C., Uhlenbruck, G. U., Shinoda, T., Shimizu, H. & Ushijima, H.,
1991. Antibodies against defined carbohydrate structures of Candida
albicans protect H9 cells against infection with human immunodeficiency
virus-1 in vitro. J. Acquir. Immun. Defic. Syndr. 4:694-703.
Panem, S., 1979. C Type Virus Expression in the Placenta.
Current Topics in Pathology 66:175-189.
Papadopulos-Eleopulos, E., Turner, V. F., Papdimitriou,
J. M., Causer, D., Hedland-Thomas, B. & Page, B., 1994. A critical
analysis of the HIV-T4-cell-AIDS hypothesis. Genetica 95:5-24.
Sinoussi, F., Mendiola, L. & Chermann, J. C., 1973.
Purification and partial differentiation of the particles of murine sarcoma
virus (M. MSV) according to their sedimentation rates in sucrose density
gradients. Spectra 4:237-243.
Snyder, H. W. & Fleissner, E., 1980. Specificity of
human antibodies to oncovirus glycoproteins: Recognition of antigen by
natural antibodies directed against carbohydrate structures. Proc. Natl.
Acad. Sci. U S A 77:1622-1626.
Strandstrom, H. V., Higgins, J. R., Mossie, K. & Theilen,
G. H., 1990. Studies with canine sera that contain antibodies which recognize
human immunodeficiency virus structural proteins. Cancer Res.
Toplin, I., 1973. Tumor Virus Purification using Zonal
Rotors. Spectra No. 4:225-235.
WHO, 1994. HIV type 1 variation in World Health Organization-sponsored
vaccine evaluation sites: genetic screening, sequence analysis, and preliminary
biological characterization of selected viral strains. AIDS Res. Hum.