In a sub-study of the ENGAGE AF TIMI-48 trial (which compared warfarin and edoxaban in atrial fibrillation patients; Mega et al., 2015) warfarin-treated participants with a sensitive or highly sensitive genotype (e.g., VKORC1 −1639AA or CYP2C9*1/*3) spent a greater proportion of time within the supratherapeutic INR range (i.e., INR >4) and had higher rates of bleeding in the initial 90 days of treatment, as compared to those with non-sensitive genotypes. The gene discussed is VKORC1; the disease is atrial fibrillation.