Three multisite clinical trials have examined the efficacy of CYP2C9 plus VKORC1 genotype-guided warfarin, of which two trials demonstrated favorable effects of a genotype-guided approach on the outcome of improved anticoagulation control or reduction in risk for bleeding, thromboembolism, death, or supratherapeutic anticoagulation following total joint arthroplasty. The gene discussed is CYP2C9; the disease is Thromboembolism.