with the potential for insertion of the mutant vector into other organs 
including the germ line. (2) If the human genome contains sequences that, at 
the time of vector insertion, are already producing the necessary products for 
virus formation ( gag , pol and erw proteins), and the products of these genes 
can recognize and package the transcribed vector, then the patient's cell could 
act as a producer cell, yielding replication-defective SAX-contalnlng virions 
that could infect other tissues including the germ line. 
What is the likelihood that recombination with endogenous sequences will 
result in a replication-competent retrovirus or that host cells can act as 
producer cells of inserted vectors? Endogenous retroviral sequences are common 
in mice: some express replication-competent retroviruses at low levels 
(Aaronson and Dunn. 1974). Recombination between replication defective type C 
retroviruses and endogenous mouse retroviral sequences has produced 
replication-competent retrovirus (Stephenson et al.. 1974b; Famulari. 1983). 
Oncogenic viruses have been formed by chemical- or radiation-induced 
recombination between non-oncogenic or weakly oncogenic retroviruses and 
endogenous viral sequences (Yanagihara et al.. 1985; Galiay et al.. 1986). 
Homologous recombination has also been shown in the mouse in a case in which 
repair of a 400 bp deletion in the pol_ region occurred along with the 
restoration of the replicative function of the virus (Schwartberg et al.. 
1985 ). 
Retroviral -1 ike sequences are located throughout the human genome. Some 
of the sequences identified have homology with the mammalian type C 
retroviruses (of which Moloney MuLV is a member), while others appear to have 
different origins. Some sequences may be present with copy numbers as high as 
1000. Various sequences have been shown to be transcribed and the level of 
mRNA transcripts appears to be higher in placenta and malignant cells than In 
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Recombinant DNA Research, Volume 12 
