MMTV-c-myc transgenic mice develop mammary adenocarcinomas in both the virgin state (∼50% incidence following a 7–14 month latency) and the multiparous state (∼100% incidence with two or more pregnancies); however, the extended mammary tumour latencies and low mammary tumour multiplicities suggest that c-myc is contributory to but insufficient for mammary tumorigenesis in the mouse (Stewart et al, 1984; Leder et al, 1986). This evidence concerns the gene MYC and breast adenocarcinoma.