Molecular genetics of congenital adrenal hyperplasia.

More than 95% of cases of congenital adrenal hyperplasia are attributable to steroid 21-hydroxylase (21-OH) deficiency. In normal individuals, there are usually two 21-OH genes on each chromosome 6, a functional 21-OH gene-CYP21B-and a closely related 21-OH pseudogene-CYP21A-which is defective in expression. Recent advances have shown that the pathologic mutations that contribute to 21-OH deficiency arise as a consequence of unequal crossover and gene conversion-like mechanisms that involve sequence interaction between the normally functional 21-OH gene and its pseudogene.