Retroviral vector constructs are described which have a 5' LTR, a tRNA binding site, a packaging signal, one or more heterologous sequences, an origin of second strand synthesis and a 3' LTR, wherein the vector construct lacks retroviral gag/pol or env coding sequences. In addition, gag/pol, and env expression-cassettes are described wherein the expression cassettes lack a consecutive sequence of more than 8 nucleotides in common. The above-described retroviral vector constructs, gag/pol and env expression cassettes may be utilized to construct producer cell lines which preclude the formation of replication competent virus.