Bloom syndrome (BS) is caused by homozygous or compound heterozygous mutations in the RecQ DNA helicase gene BLM. Since the molecular isolation of BLM, characterization of BS-causing mutations has been carried out systematically using samples stored in the Bloom's Syndrome Registry. In a survey of 134 persons with BS from the Registry, 64 different mutations were identified in 125 of them, 54 that cause premature protein-translation termination and 10 missense mutations. In 102 of the 125 persons in whom at least one BLM mutation was identified, the mutation was recurrent, that is, it was shared by two or more persons with BS; 19 of the 64 different mutations were recurrent. Ethnic affiliations of the persons who carry recurrent mutations indicate that the majority of such persons inherit their BLM mutation identical-by-descent from a recent common ancestor, a founder. The presence of widespread founder mutations in persons with BS points to population genetic processes that repeatedly and pervasively generate mutations that recur in unrelated persons.