A series of retrovirus based vectors have been developed for the efficient transmission of genetic information in mammalian cells. The vectors are based on the murine MuLV virus. The v-ras gene of Harvey murine sarcoma virus (HaMuSV) has been utilized as a convenient gene for functional expression. Expression of transmitted information has been rendered conditional by linkage of the p21 coding sequence to the glucocorticoid regulated promoter from mouse mammary tumor virus (MMTV).