We have produced 13 strains of transgenic mice that carry an otherwise normal mouse myc gene in which increasingly larger portions of the myc promoter region have been replaced by a hormonally inducible mouse mammary tumor virus promoter. Although expression of the fusion genes varies among these animals, the female founders of two of these transgenic strains spontaneously developed mammary adenocarcinomas during one of their early pregnancies. Both the tumors and the breast tissue of these founder animals expressed RNA transcripts corresponding to the fusion gene. Furthermore, in the best studied strain, all the available F1 female progeny that inherited the MTV/myc gene also developed mammary adenocarcinomas during their second or third pregnancies. Thus, although it has no obvious effect on the early development of these mice, the constitutionally deregulated myc gene appears to act as a heritable, predisposing factor favoring the accelerated development of a tissue-specific adenocarcinoma.