13 
Dr. Mulligan said the basis for the selection of specific membrane proteins 
for inclusion in a retrovirus particle is not clear, although the viral gag 
proteins presumably play some role in the process. 
Dr. Joklik said human cells are not thought to produce envelope proteins. 
Endogenous sequences for such a protein have been identified in the human genome 
but do not appear to be expressed normally. 
Dr. Gottesman asked Dr. Hartley whether murine cells express membrane proteins 
which are incorporated into retroviral coats. Dr. Hartley said seme mouse 
cell lines express proteins which cculd be incorporated into retroviral coats. 
Dr. Gottesman asked whether there are pseudotypic combinations of retroviral 
gencmes with amphotropic envelope proteins which might present a hazard; e.g., 
amphotropically packaged retroviruses carrying an oncogene. 
Dr. Simpson said sane work had been done with mice which suggests that one 
round of replication of a retrovirus carrying an oncogene is sufficient to 
cause tumors. 
Dr. Liberman said the MIT Biosafety Office is not concerned that an amphotrop- 
ically packaged replication defective retrovirus might cause an epidemic; the 
office is concerned, however, that amphotropic retrovirus vectors carrying an 
oncogene may present a risk of carcinogenesis to laboratory personnel. MIT 
requires BL3 containment for the first two or three rounds of replication of 
these agents in order to protect personnel. Containment may be lowered to BL2 
for subsequent rounds of replication. Dr. Mulligan said his laboratory uses 
BL3 containment for research involving amphotropic retroviral vectors carrying 
an oncogene. 
Dr. Gottesman noted that the situation of laboratory workers differs frem 
the situation of the general public in that laboratory personnel would be 
exposed to the initial rounds of replication of these agents while the public 
would not be expected to be exposed to these initial rounds. 
Dr. Gottesman asked whether special techniques were used in the laboratory 
in handling the murine retroviruses. Dr. Hartley replied that the good laboratory 
techniques specified for working with infectious agents are sufficient; under 
those conditions, there is no evidence of infection of laboratory workers. 
Dr. Mulligan asked whether researchers employed special precautions in handling 
feline leukemia virus. Dr. Hartley said her laboratory employs the procedures 
recommended in the NIH Guidelines for handling infectious agents. The helper 
virus used in procedures with feline leukemia virus is not particularly infect i i,s 
for humans, although it can produce lymphomas in mice. 
Dr. Friedman asked whether chromosomal transposition occurs more frequently 
with retrcviral proviruses than with DNA viruses. 
Dr. Joklik said there are no specific chromosomal sites for viral integration, 
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