An unequal crossover event in RCCX modules of the human MHC resulting in the formation of a TNXB/TNXA hybrid and deletion of the CYP21A.

The central region of the human major histocompatibility complex contains tandemly arranged genes of RP, C4, CYP21, and TNX. The C4 gene region is prone to rearrangements that generates duplications, conversions, and deletions. Diversity in gene number and size causes reorganization and may lead to genetic disorders. The RP, C4, CYP21, and TNX genes form a genetic unit called RCCX. We describe molecular studies on RCCX haplotypes revealing a unique recombination giving rise to a TNXB/TNXA hybrid gene, CYP21A deletion and CYP21B duplication on one chromosome of the propositus. His other chromosome carries a deletion of CYP21A-TNXA-RP2-C4B genes, resulting in the total absence of CYP21A genes and the presence of three CYP21B genes in the genome.