CYP21A2 and classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency: As mentioned above, chimeric CYP21A1P/CYP21A2 genes are caused by homologous recombination between CYP21A2 and its pseudogene CYP21A1P as a result of unequal crossover and are found in 20-25% of alleles in CAH due to 21-OHD.