Thrombin generation in plasmas containing FV_R2 and/or FV_Leiden. Thrombin generation after extrinsic activation of plasma in the absence (filled symbols) and presence (open symbols) of 5 nM APC was measured as described in “Materials and methods.” (A) Normal pooled plasma. (B) Plasma of an FV_R2-homozygous individual. (C) FV_Leiden-heterozygous plasma obtained by pooling plasma from 2 unrelated FV_Leiden-heterozygous individuals. (D) Simulated FV_Leiden/FV_R2-doubly heterozygous plasma obtained by mixing plasma from an FV_Leiden-homozygous individual and from an FV_R2-homozygous individual in equal proportions.