Our results demonstrating that p18 depletion rescues the proliferative defects induced by haploid loss of Gata3 and that p18;Gata3 double mutant mice develop basal-like mammary tumors suggest that depletion of p18 is required for proliferation of GATA3 defective cells and for development of GATA3 deficient basal-like mammary tumors. The gene discussed is CDKN2C; the disease is breast cancer.