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A high incidence of discoid lupus erythematosus (DLE) and urticaria/angioedema was found among patients with selective low levels of the C4 component of complement. Although evidence for activation of C4 was present in patients' sera, studies to determine the presence of known activation mechanisms were negative. Genetic C4 typing in four pedigrees showed that all propositi carried the null allele B*QO. It is postulated that the low C4 levels in these patients are related to the skin lesions and this partial genetic deficiency.