Hsien-Hsiung Lee
A defective CYP21A2 gene downstream of the TNXB gene in congenital adrenal hyperplasia (CAH) falls into three categories: (a) small-scale conversions of CYP21A1P, (b) spontaneous mutations, and (c) chimeric RCCX modules that include the chimeric CYP21A1P/ CYP21A2 and TNXA/TNXB genes [1]. Most of the CYP21A2 mutations identified so far were a result of small-scale conversions of the CYP21A1P (up to 11 for CYP21A1P) during both meiosis and mitosis [2], which account for about 70%-80% of all CAH cases. The Chimeric CYP21A1P/CYP21A2 and TNXA/TNXB genes, which result from unequal cross-over (or deletions) during meiosis [2] and occur in ~20% of CAH alleles in most populations [1,3] respectively reflect the deletion of the 1/XCYP21A1P - XA - RP2 - C4B - 1/XCYP21A2 gene array (1/X indicates an uncertain fraction of the gene sequence) [1] and a deletion of the RP2 - C4B - CYP21A2 - 1/XTXNB gene array [1]. Their deletion range is about a 26- or 32-kb gene sequence which depends on whether C4B is the long or short gene (more commonly shown in the literature as being 30 kb). In fact, these different types of large-gene deletions in the RCCX region are generally considered to represent one event in many studies.
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