The breast and ovarian cancer susceptibility gene product BRCA1 maintains genome stability mainly through homologous recombination repair (HR) functions against DNA breaks. While failure of this function causes carcinogenesis, it is also important as a target for treatment with poly (ADP-ribose) polymerase (PARP) inhibitors that induce synthetic lethality with the HR deficiency. In variants of unknown significance (VUSs), the HR capacity has been analyzed as a functional assessment to predict the risk of developing cancer. Thus, the assessment of HR function in BRCA1 is important for both carcinogenesis prediction and therapy. Pathological missense mutations in BRCA1 are concentrated in the N-terminal RING finger domain and the C-terminal BRCT domain. The RING finger of BRCA1, together with its structurally similar partner BARD1, forms a RING heterodimer with E3 ubiquitin ligase activity. Since its discovery more than 20 years ago, the biological and clinical significance of this enzyme activity has gradually become clear. In this chapter, the authors describe what is currently known about the E3 activity of BRCA1, which is important for carcinogenesis and PARP inhibitor sensitivity.