Given the emergence of BH3-mimetic drugs capable of targeting MCL-1 we investigated the expression and functional requirement for MCL-1 in breast cancer, systematically testing this through a combination of human breast tumour tissue analysis with correlation to clinicopathological data; breast cancer cell line testing in vitro and in vivo; and for the first time show a role for MCL-1 in mammary tumorigenesis using a genetically engineered mouse model. The gene discussed is MCL1; the disease is breast cancer.