Familial discoid lupus erythematosus associated with heterozygous C4 deficiency.

Low levels of the fourth component of complement (C4) were found functionally and immunochemically in two sisters (17 and 22 years of age) suffering both from discoid lupus erythematosus. C4-typing of the family revealed that the two sisters and the mother had only two, and the father three of the four genes coding for C4 expressed. All carried the null allele C4B*QO. This observation adds to the increasing evidence that not only complete but also partial genetic complement deficiencies may predispose to lupus erythematosus.