(A) The phage ϕC31 uses an integrase-driven system to insert its genetic content into a host bacterium. The system requires ϕC31 integrase and a pair of complementary sequences in the phage (attP) and the bacterial host genome (attB). (B) Two plasmids, one containing ϕC31 integrase, the other containing the attP site and the therapeutic transgene, are used to co-transfect mammalian cells. The ϕC31 integrase effects transgene insertion into the mammalian genome at pseudo-attB sites, recognized by the attP site in the therapeutic gene plasmid.