CDKN2A and breast cancer: Notably, Pdgfrβ and Ck14 doubly positive tumor cells were also detected in lung metastases of p18−/−;Brca1MGKO and p16−/−;Brca1MGKO mammary tumors (Fig. 2a, b, and Additional file 3), which demonstrated that the metastases were derived from Brca1-deficient basal-like mammary tumors with an increased Pdgfrβ expression.