BRCA1 is a putative tumour suppressor gene located on chromosome 17q21. It spans 100kb of genomic DNA and encodes a protein of 200kD consisting of 1863 amino acids. Sixty-three distinct germline mutations of BRCA1 have now been identified in more than 100 patients with breast and/or ovarian cancer. These mutations are distributed across the entire coding region of the BRCA1 gene, and the majority (87%) are predicted to result in truncated proteins or loss of a BRCA1 transcript. No somatic mutations of the BRCA1 gene have been identified in sporadic breast cancers, though five mutations have been found in sporadic ovarian tumours. This suggests that mutations in the BRCA1 gene may play a significant role in the tumorigenesis of familial breast cancer but not of sporadic breast cancer.