Genetic basis of acquired C4 deficiency.

A study of the family of a patient who had an SLE-like syndrome and an extremely low serum C4 revealed an inheritance of C4 types and HLA region markers which indicated that the patient had 60--70% of "normal" C4 level prior to the onset of disease. Thus the extremely low C4 level during her disease may result from a combination of genetically determined low normal C4 and increased consumption/hyposynthesis secondary to her SLE.