Cystic fibrosis (CS) is a Mendelian recessive disorder and only appears when both alleles are mutated. The gene responsible for cystic fibrosis is Cystic Fibrosis Transmembrane Conductance Regulator Gene (CFTR), and codes for chloride channels in the apical membrane of epithelial cells. The protein consists of two sets of six membrane spanning segments, two nucleotide-binding domains (NBD1 and NBD2) and a regulatory domain (R) [1]. More than 1000 different mutations and 200 polymorphisms have been reported in this gene (www.genet. sickkids.on.ca), making CF one of the best examples of allelic heterogeneity in human genetic diseases.

       Different mutations in protein regions have different effects on its function. However, not only mutations can change the protein function. For example, a polymorphic T_n locus, with five or seven T-nucleotide repeats, in front of exon 9, can cause a downsynthesis of a full-length CFTR protein [3,4]. Therefore, it can be concluded that polymorphisms, in combination with mutations, may also be responsible for variation in CF phenotype expression [2].

      In this paper we report on two different alleles (found in two unrelated patients from Croatia), where the same mutation was combined with different polymorphisms. The polymorphism F508C is represented as a missense substitution T®G at nucleotide position 1655, which changes the protein’s phenylalanine residue to cysteine (protein function is retained). The polymorphism P1290P is characterized by the substitution of nucleotide A with G in the second codon (out of two) for phenylalanine in exon 20 (amino acid position 1290).