One study has reported a potential association between low-activity alleles (UGT1A1*28 and UGT1A1*34) and increased risk of breast cancer in premenopausal women of African ancestry (odds ratio [OR] = 1.8, 95% confidence interval [CI] = 1.0–3.1, P = 0.06), and the association was strongest in ER-negative breast cancer (OR = 2.1, 95% CI = 1.0–3.1, P = 0.04). Here, UGT1A1 is linked to breast cancer.