The RD (D6S45) gene in the class III region of the HLA major histocompatibility complex encodes a protein normally containing 24 consecutive basic-acidic dipeptide repeats. We determined the frequency of variations in the number of repeats by use of the polymerase chain reaction. Of 107 subjects 7 (3.3%) carried genes encoding 22 or 23 repeats. There was no difference in the frequency of such polymorphisms between normal individuals and those with systemic lupus erythematosus, a disease associated with other polymorphisms in the class III region of HLA. The frequency of polymorphisms in proteins with oligopeptide repeats may provide useful information concerning functional constraints on repeat number.
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