Human studies have suggested that high-activity CYP1B1 alleles and low-activity COMT alleles are associated with increased risk of breast cancer (Wen et al. 2007) and other E2-responsive cancers (Nock et al. 2006; Sellers et al. 2005; Zimarina et al. 2004). This evidence concerns the gene CYP1B1 and breast cancer.