C4 Haplotypes with Duplicated C4A or C4B: Frequency and Associations with Bf, C2, and HLA-A, B, C, DR Alleles with Special Reference to the Duplication C4B1,2

Abstract

The fourth component of human complement (C4) is encoded by two separate genes, C4A and C4B, located between the HLA-B and HLA-DR loci [4]. The recent technology of C4 typing has allowed the identification of 12 alleles at the C4A locus and 18 alleles at the C4B locus [12]. In addition, “null alleles”, AQO and BQ0, are relatively common at both loci. Duplications at C4A or C4B have also been described [2, 3, 7, 11, 13, 16, 18], and it has been proposed that they have arisen by unequal crossing-over between chromosomes with two expressed genes [1].