The  retrovirus  Moloney  murine  leukemia  virus (MLV) is a candidate
   vector for gene therapy.
<lI> Three genes are removed to prevent replication:
gag encodes viral coat proteins,
pol  is  an  RNA-dependent DNA polymerase (Reverse Transcriptase),
and
env encodes surface glycoproteins.
<lI> The  SAX  vector  retains  the  long  terminal  repeat (LTR) and q
sequences  to  allow  transcription  and integration into the host
genome, but also includes these:
neo^r confers neomycin resistance as a selective marker, and
SV40 promoter-enhancer fused with
hADA, a normal adenosine deaminase gene.