The transgene comprises approximately 80 copies of the genome of the shuttle vector lambda EG10, which is composed of the genome of phage lambda2001 joined to a linearized plasmid. Lambda2001 contains, in addition to the genes necessary for infectivity, a chi site and the red and gam genes that together confer sensitivity to P2 interference. The plasmid portion of EG10 contains two loxP sites and the E. coli chloramphenicol resistance (CamR) and xanthine-guanine phosphoribosyl transferase (gpt) genes.