Abstract
Expression of the human endogenous retrovirus genome HERV-K, homologous to mouse mammary tumor virus, was investigated in cultured human tumor cells. In several cell lines, the HERV-K genome was expressed as an 8.8-kilobase poly(A)+ RNA which appeared to be a full-size transcript of this genome. In the human breast cancer cell line T47D, stimulation of HERV-K genome expression by progesterone was observed after estradiol treatment.