Neither the CYP2C19*17 variant, which results in a rapid metabolizer phenotype, nor the CYP2C19*2 variant were associated with breast cancer risk in population-based studies of women of European descent; however, CYP2C19*17 was associated with decreased risk in women using hormone replacement therapy for >10 yr (Justenhoven et al. 2009, 2012; MARIE-GENICA Consortium on Genetic Susceptibility for Menopausal Hormone Therapy Related Breast Cancer Risk 2010). This evidence concerns the gene CYP2C19 and breast cancer.