Evidence exists that in animals of some species treated with estrogens the levels of viral (or viral like) proteins in uterine tissue increase, and that these levels vary with the estrous cycle and the gestational stage of the animal. Experiments using an estrogen radioimmunoassay will be performed to correlate tissue levels of estrogen to the degree of expression of such viral proteins. DNA-RNA hybridization will be employed to determine if estrogens modify the transcription and/or translation of the viral genes coding for such proteins. The site of action of the estrogens in the nucleus will also be explored by investigating their association with the reiterated or unique sequences of the cellular genome of the early embryo and of the mature animal. Attention will be directed to the presence of viral genes in the sequences of the cellular genome which contain estrogen- binding sites.