sentences:  The homogeneity or heterogeneity of the two loci, with the addition of these null (f0, s0) genes, allow for duplication/non-duplication of the C4 loci.Therefore, having separate loci for C4, C4F and C4S (later identified as C4A or C4B, respectively), possibly account for producing multiple allelic forms, leading to the great size and copy number variation.Two important contributors, Carroll and Porter, in their study of cloning the human C4 gene showed that all six of their clones contained the same C4 gene.