sentences:  The size of either C4A or C4B gene can be 21 kb (long, L) or 14.6 kb (short, S).Also, the long C4 gene uniquely contains a retrovirus HERV-K(C4) in its intron 9 that imposes transcription of an extra 6.36 kb, hence the “longer” string of gene.Thus, C4 genes have a complex pattern of variation in gene size, copy number, and polymorphisms.