Transcription of cellular sequences flanking proviral insertion sites was studied in several Mov mouse strains, each of which carried one copy of the Moloney murine leukemia virus in its germ line. In three out of five randomly chosen Mov strains, the provirus had integrated in the vicinity of DNA regions transcribed in the embryonal stem cell line CCE and the embryonal carcinoma cell line F9. Assuming that CCE and F9 cells are developmentally equivalent to the early embryonic cells that were infected to establish the Mov strains, our results suggest that retroviruses integrate preferentially into actively transcribed DNA regions.