Here, we used a Bayesian pathway modeling approach to investigate whether genetic variant interactions within key enzymes and transporters known to be involved in tamoxifen metabolism—including CYP2D6, CYP3A4, CYP2C19, and efflux and uptake transporters—might influence treatment discontinuation rates among a cohort of premenopausal women with breast cancer. The gene discussed is CYP2D6; the disease is breast cancer.