CFTR and cystic fibrosis: Three patients where 1 allele remained undetected have classical form of CF with mean sweat chloride concentrations over 60 mM which shows that pathogenic CFTR variants may be present in nonexamined CFTR introns or that there are other molecular mechanisms involved, but not covered by the utilized assays and/or bioinformatic algorithms (Chen et al., 2008; Lee et al., 2017).