This proposal is designed to study the introduction of new DNA sequences into stem cells of two developmental systems: the mouse teratocarcinoma system and the mouse pre-implantation embryo. We propose to introduce new regulatory sequences and genes into these cells and then follow their effects upon the host genome during differentiation and embryonic development. The means by which we propose to do this is by using retroviral vectors which have new regulatory sequences and genes introduced into their proviral forms. We also propose a means for efficiently selecting the integration sites of the retroviral vectors so that we can examine the effects of the newly introduced sequences upon the cellular sequences adjacent to their integration sites. The work will proceed with three specific aims: 1) To examine the nature of the blocks to Moloney murine leukemia virus (M-MuLV) gene expression in the stem cell, embryonal carcinoma (EC), of teratocarcinomas. 2) To construct and use M-MuLV recombinants as vectors for introducing new regulatory sequences and structural genes into EC cells and mouse pre-implantation embryos. 3) To isolate the integration sites of M-MuLV vectors introduced into EC cells and embryos; and to examine whether the vectors integrate into active or inactive regions of the genome and, or, whether newly introduced enhancing sequences can activate genes adjacent to the proviral integration site.