The ubiquitin ligase activity of the breast and ovarian cancer-associated tumor suppressor protein BRCA1, as part of a heterodimer with BARD1 (BRCA1-associated RING domain 1), is critical for its role as a tumor suppressor, but its targets, and the consequences of ubiquitin modification, by the BRCA1 protein complex are only now being discovered. BRCA1 is now known to control the ubiquitination of the estrogen and progesterone receptors, and although the consequences of the ubiquitination of these steroid receptors are unknown, the identification of these specific substrates suggests a link between the ubiquitin ligase activity of BRCA1 and its specificity as a tumor suppressor in breast and ovarian tissues.