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The L138ins mutation, found in Russian cystic fibrosis (CF) patients, is a duplication of three nucleotides (CTA) in exon 4 of the CFTR gene and is categorised as a small in-frame insertion/deletion. As a result, the CFTR protein molecule elongates by one amino acid residue, leucine, at position 138 (codon 138 (CTA)). In accordance with the new nomenclature, it should be called c.411_412insCTA (p.Leu138dup). The c.411_412insCTA (p.Leu138dup, L138ins) mutation is found in CF patients of Slavic origin (Russians, Ukrainians) and has been linked to a single haplotype of the intragenic DNA markers IVS1CA-IVS6aGATT-IVS8CA-IVS17bCA - 22-7-16-13.