IN 1945 Foulds mated some C57/BL female mice with RIII males. Some of the progeny of these matings developed mammary tumours and the sixth female to do so and a male of the same litter constituted the parents of the BR6 strain which has been maintained by brother-sister mating ever since. Foulds supposed that the BR6 strain harbours the mammary tumour virus derived from its RIII male progenitor since the virus was demonstrable in a similar C57 x RIII hybrid bearing mammary tumours (Foulds, 1949a). The tumour incidence in the strain was high from the start (Foulds, 1949a). The tumours are influenced greatly by hormonal conditions; many of them in the original hybrids were readily transplantable into female recipients of the same genetic constitution or into males treated with oestrogen but grew slowly after a long latent period, or not at all, in males (Foulds, 1947), though this sex factor in transplantation tended to be lost in subsequent generations (Foulds, 1949b). The most striking characteristic of the tumours in the BRG line was their pregnancy dependence. The tumours almost invariably first appeared during pregnancy
