Genetic heterogeneity of factor XIII deficiency: first description of unstable A subunits.

In previously reported cases, congenital deficiency of coagulation factor XIII has been found to result from the absence of the enzymatically active A subunits. In the family reported here two members were found to be heterozygous for an unstable A subunit which, in the homozygous state, or in the heterozygous state with the previously described null allele, would be expected to result in factor XIII deficiency. The implications that this new variant has on heterozygote screening are discussed.