Methods.

Subjects.

Sixty-four family members (34 of whom were patients) from six pedigrees had blood drawn. Data from Families A, B, C, and D have previously been published.[1, 6] Genealogical investigation could not establish links between the first three pedigrees from the beginning of the eighteenth century onwards. No proper genealogical studies have been undertaken for Families D, E, and F. Pedigree D is of Polish descent and the other families are not known to be related to each other. Except for two new patients, all patients had previously been seen at our department. Variability in severity of weakness as well as nature and site of contractures is present within and between families. Individuals were considered affected on the basis of the presence of weakness and/or contractures. The clinical characteristics are summarized for each family in Table 1.