Complement C4 is one of the components in the intricate complement system, which is encoded by two closely linked genes (C4A and C4B) located within the major histocompatibility complex (MHC) on chromosome 6p21.3. Because the C4A-C4B genes are two-locus allelic model, the genetics of human complement C4 is characterized with an abundant variation in the gene copy number and gene size within a population. Learn more: complement component c4