Chronic immune thrombocytopenic purpura (ITP) appeared in monozygotic twin girls during adolescence. An older sister was discovered to have less severe thrombocytopenia without purpura. Increased amounts of platelet-associated immunoglobulin were detected in the proband and her sisters but in no other members of the family. Antibodies reacting with nuclear components were demonstrated in the serum of the three sisters, both parents, and other members of the kindred, some of whom had immunologically mediated clinical disorders. These observations add to the increasing evidence that a genetically determined disorder of immune regulation is often a factor predisposing to chronic ITP. The Kindred.
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