Deficiency of the fourth component of complement (C4): a family case.

In this report, an apparently healthy 38-year-old woman with a remarkably low serum C4 value is described together with other family members who had moderately low serum C4. Plasma C4 typing disclosed that the proband inherited two C4B "null" haplotypes. In addition, Southern blot analysis of the C4 gene indicated that the C4A gene was partially deleted on one of these two haplotypes in the proband. We thus concluded that a de novo deletion on the inherited half-null haplotype was the likeliest cause of the low C4 level.