Factor XI deficiency: genetic and clinical studies of a single kindred.

A four-generation 25-member kindred with Factor XI:C deficiency is reported. Factor XI:C levels in heterozygotes varied from 15 to 58%, suggesting that Factor XI:C values for homozygote determination should be less than 15%. The frequency of bleeding was not correlated with Factor XI:C levels in this range. Individuals with joint pain had significantly lower Factor XI:C levels than members without joint pain and pain occurred more frequently in frequent bleeders. Lod scores showed no close genetic linkage of Factor XI:C deficiency with blood group MNSs (chromosome 4), complement components Bf and C4B (chromosome 6), or blood group P.