Inheritance of a mutation in BRCA1 (breast cancer 1 early-onset) results in predisposition to early-onset breast and ovarian cancer. Tumours in these individuals arise after somatic mutation or loss of the wild-type allele. Loss of BRCA1 function leads to a profound increase in genomic instability involving the accumulation of mutations, DNA breaks and gross chromosomal rearrangements. Accordingly, BRCA1 has been implicated as an important factor involved in both the repair of DNA lesions and in the regulation of cell-cycle checkpoints in response to DNA damage. However, the molecular mechanism through which BRCA1 functions to preserve genome stability remains unclear. In the present article, we examine the different ways in which BRCA1 might influence the repair of DNA damage and the preservation of genome integrity, taking into account what is currently known about its interactions with other proteins, its biochemical activity and its nuclear localization.