The retrovirus Moloney murine leukemia virus (MLV) is a candidate vector for gene therapy.

1. 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.

2. The SAX vector retains the long terminal repeat (LTR) and ψ 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.