Alterations in p53 and Rb tumor suppressors or their pathways are common in human breast cancer. We used newly generated MMTVCre105Ayn mice to inactivate p53 and/or Rb strictly in the mammary epithelium. Inactivation of p53 led to formation of estrogen receptor positive raloxifene-responsive mammary carcinomas with features of luminal subtype B. Rb deficiency was insufficient to initiate carcinogenesis but accelerated formation of mammary carcinomas in combination with p53 inactivation and promoted genomic instability. The recurrent amplification of the cIAP1, cIAP2 and Yap1 protooncogenes in the 9A1 locus was observed only in neoplasms associated with p53 inactivation alone. However, all three genes remained overexpressed in carcinomas with p53 and Rb inactivation due to elevated E2F expression and cooperated in carcinogenesis. To test involve ment of the stem cell compartment in the initiation of mammary carcinogenesis we have isolated mammary stem cells (MRU; CD24medCD49fhigh), and their progeny (Ma-CFC, CD24highCD49flow). Loss of either p53 or Rb led to an increase in the number and proliferation rate of Ma-CFC but not MRU. Both parameters were further increased in Ma-CFC pool deficient for both p53 and Rb. Notably, the resulting mammary neoplasms contained CD24+CD49f+ cells. Far fewer of CD24+CD49f+ cells were required for orthotopic transplantation, as compared to CD24-CD49f- cell population. CD24+CD49f+ cells yielded carcinomas much earlier and were able to reconstitute all tumor cell populations more efficiently than CD24-CD49f- cells in secondary and tertiary transplantations. Thus, CD24+CD49f+ cells meet the definition of cancer stem cells (CSC). We have determined that CSC show higher genomic stability and are characterized by upregulation of Aldh1, cIAP1 and downregulation of microRNAs miR-376b and miR-467b. Importantly we have identified cIAP1 and cIAP2 as targets of both miRNAs and shown that miR-376b, miR-467b, cIAP1 and cIAP2 affect stem cell properties of CSC. Taken together, these findings establish a model of luminal subtype B mammary carcinoma, demonstrate that p53 and Rb cooperate in the maintenance of genomic integrity, identify critical role of cIAP1, cIAP2 and Yap co-expression in mammary carcinogenesis. Furthermore, our studies identify and characterize CSC and uncover miR376b/miR-467b - cIAP1/2 pathway as a promising novel target for CSCspecific therapy.