Congenital adrenal hyperplasia (CAH) is a group of autosomal recessively inherited disorders characterized by impaired production of adrenal steroids. Approximately 95% of all CAH are caused by mutations of the CYP21A2 that encodes 21-hydroxylase. In this study, mutation analyses of CYP21A2 were performed in 48 CAH patients from 45 Turkish families with the clinical diagnosis of 21-hydroxylase deficiency (21OHD). While in 39 (86.7%) of 21OHD patients, disease causing CYP21A2 mutations were identified in both alleles, in two 21OHD patients CYP21A2 mutations were identified only in one allele. In four patients, mutation was not detected at all. In total, seventeen known and one novel, disease causing CYP21A2 mutations were observed. Among identified mutations, previously described c.293-13C/A>G, large rearrangements and p.Q319X mutations were the most common mutations accounting for 33.3%, 14.4% and 12.2% of all evaluated chromosomes, respectively. In six families (13.3%) a novel founder mutation, c.2T>C (p.M1?), inactivating the translation initiation codon was found. This mutation is not present in pseudogene CYP21A1P and causes the classical form of the disease in six patients. In addition, depending on the nature of the rearrangements CYP21A1P/CYP21A2 chimeras were further classified as CHc/d, and CH-1(c) was shown to be the most prominent chimera in our study group. In conclusion, with this study we identified a novel founder CYP21A2 mutation and suggest a further classification for CYP21A1P/CYP21A2 chimeras depending on the combination of junction site position and whether it is occurred as a result of deletion or conversion. Absence of disease causing mutation of CYP21A2 in ten of screened ninety chromosomes suggests the contribution of regulatory elements in occurrences of CAH due to the 21OHD. (C) 2012 Elsevier B.V. All rights reserved.