AR and breast cancer: Using an androgen receptor knockout (ARKO) androgen-insensitive female mouse model (expressing a mutated, inactive AR) combined with DMBA chemical carcinogenesis, however, we demonstrated that the onset of palpable mammary tumors was significantly faster in ARKO females compared with wild-type female mice (median time, 22?weeks vs 34?weeks, respectively) [1], supporting the inhibitory role of AR-mediated androgen actions in mammary carcinogenesis.