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.     ///    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.. 