Breast cancer is one of the most common causes of cancer-related mortality in women. It affects more than one in nine women over their lifetime. Although most breast cancers are sporadic, the genetics and molecular biology of the heritable forms of breast cancer have provided valuable insights into not only breast cancer but also cancers in general. Among the mutations linked to heritable breast cancers, the mutations in the breast cancer 1 (BRCA1) gene are the best characterised. The BRCA1 gene encodes a nuclear protein that is important for maintaining genome integrity. However, a growing list of BRCA1-associated proteins suggests that BRCA1 has diverse and unexpected functions.