Analysis of the mammary tumors that arose from transplantations of pre-neoplastic mammary donor tissue from WEPtn, WB1P and WB1P-Myc mice in En1-Cre;mTmG mice revealed an increase in En1+/CD26− (En1+) and EN1−/CD26− (double-) fibroblasts, a decrease in EN1−/CD26+ (CD26+) fibroblasts and similar levels of EN1+/CD26+ (double+) fibroblasts in tumors compared to controls (Fig. 7E–G). The gene discussed is MYC; the disease is breast cancer.