Factor Ix And Factor Xi, Combined Deficiency Of

Soff et al. (1981) studied 2 families with combined deficiency of factors IX and XI. In the first family, 2 sisters had lower levels of factor IX than their mildly affected brother--findings inconsistent with X-linked recessive hemophilia B. In the second family, an 8-year-old boy with Down syndrome had factor IX levels (29-45%) higher than observed in hemophilia B. The authors were uncertain of the mode of inheritance but favored autosomal dominance. No other reported cases are known and the mechanism of the combined deficiency is unclear.