The association between RRAS2-induced breast cancer and pregnancy was also manifested by the fact that 38% of tumors were detected in Rosa26-RRAS2fl/fl x Sox2-Cre female mice that were either pregnant or breast-feeding and were significantly younger than female breeders whose tumors were detected weeks after weaning (post-partum, Fig. 1g). This evidence concerns the gene SOX2 and breast cancer.