CFTR and cystic fibrosis: The distribution of CFTR mutations in the Chechen CF population is unique in terms of the high frequency of mutations c.1545_1546delTA (p.Tyr515X; 1677delTA) and c.274G > A (p.Glu92Lys, E92K), which account for more than 90% of the mutant alleles in the studied ethnic group.