The similar mammary tumour potentials in virgins and breeders of SHN mice.

Spontaneous mammary tumour potential was compared between virgins and breeders of a high mammary tumour strain of SHN mice. Virgins were essentially similar to breeders in both the cumulative and the monthly incidences of mammary tumours. DNA synthesizing enzyme activities and the number and the weight of mammary tumours also differed little between groups. While the number of preneoplastic mammary hyperplastic alveolar nodules (HAN) was significantly higher in breeders than in virgins, little difference was observed between groups in the end-bud formation and the duct growth of normal glands and the growth of HAN. These results indicate that in SHN mice virgins have similar mammary tumour potential to breeders, which is much advantageous for practical use of this strain as an animal model for human breast and other types of cancers.