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:
    neor confers neomycin resistance as a selective marker, and
    SV40 promoter-enhancer fused with
    hADA, a normal adenosine deaminase gene.