normal cells (Robson et al.. 1985: Gattoni-Cel 1 i et al.. 1986). Antigenically 
cross-reactive proteins that complex with primate retroviral antibodies are 
expressed in normal placenta and their physicochemistry is similar to murine 
and primate type C retroviruses (Jerabek et al.. 1984). A protein found in 
human milk has been shown to have homology with the protein sequences of the 
murine mammary tumor virus (MMTV) gp55 envelope glycoprotein (Dion et al., 
1980). Antigens related to the gag specific antigen of the MMTV have been 
Identfiied In human breast carcinoma (Mesa-Tejada et al.. 1978; Ohno et al.. 
1979). The level of a glycoprotein with pl5E antigenic determinants and a 
portion of reverse transcriptase found In normal blood is Increased in patients 
with leukemia and other hematologic disorders (Jacquemin et al., 1984). 
Budding C type particles can be seen by electron microscopy of normal human 
placenta (Kalteu et al.. 1973). Despite the frequent occurrence and expression 
of human endogenous retroviral (HER) sequences In the human genome, there is 
no evidence to date that HER sequences can produce Infectious viral particles. 
The HER genomes sequenced to date contain numerous deletions or frameshift 
mutations (Bonnev et al . . 1982; Repaske et al.. 1985; Robson et al. 1985). 
suggesting that even though the genes are expressed the gene products are 
defective and not capable of forming infectious viral particles. This 
assumption Is supported by unsuccessful attempts to Isolate endogenous 
retroviruses from humans. 
Another factor to consider is the relative likelihood for recombination. 
The homology between the Moloney LTR and known HER is very low (except for 
portions of the pol region which are deleted in the SAX vector), so that 
recombination would be unlikely. Even if there were areas of the human genome 
with very similar sequences, homologous recombination in mammalian cells is an 
infrequent event, occurring with an estimated frequency of 1 x 10 to 1 x 10 
(Shaul et al . . 1985) . 
Recombinant DNA Research, Volume 1 2 
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