Summary
2 sisters of English-American descent had a mild bleeding syndrome due to marked deficiency (less than 1 % activity) of factor XI. This defect was transmitted in an autosomal recessive manner. Although factor XI deficiency was previously thought to occur largely, if not exclusively in Jews, extensive review of geneologic records and analysis of family names failed to disclose Jewish ancestry. These findings, together with the existence of several definite and presumed consanguineous English-American ancestors, the fact that family members had resided in a restricted geographic area for many generations, and analysis of English and Jewish immigration patterns lead to the conclusion that this defect is not likely to be of Jewish derivation. Should this mutation have occurred in the distant past it is conceivable that the gene pool for this defect is substantial, particularly in certain areas in New England.
