F8 mRNA analysis. Total RNA from peripheral blood lymphocytes was reverse transcribed with an oligo-(dT) primer, and F8 exon 22 was amplified in carriers of the F8 duplication from both families (coded as in Figure 1) and in 4 normal controls. The relative expression of F8 mRNA in carriers of the F8 duplication vs normal controls was calculated using ACTB and GAPDH as reference genes. Error bars represent standard deviations of at least 3 determinations. The dashed line represents the normal F8 expression level in controls.