**1. Introduction**

The autosomal recessive mode of inheritance of Cystic Fibrosis (CF) was suggested by Andersen and Hodges in 1945 when they described this disease as a pathological entity.

Since the discovery of the *CFTR* gene (and the predicted protein cystic fibrosis transmembrane conductance regulator) in 1989 [1], close to 2, 000 variations have been identified on this locus. *CFTR* gene studies represent one of the most frequent genetic analyses routinely performed worldwide, either to confirm the clinical diagnosis of CF or CFTR-related disorders (CFTR-RDs), or to offer carrier testing, prenatal or pre-implantation genetic diagnosis.

The most common severe mutation, p.Phe508del, is found in approximately 70% of CF alleles of European descent (therefore it is present in 49% of homozygous and 42% of compound heterozygous CF patients), with significant variations depending on ethnicities [2]. There is a clear decreasing northwest to southwest gradient in p.Phe508del frequency across Europe. Only four other mutations represent more than 1% of CF cases: p.Gly542\*, p.Gly551Asp, p.Asn1303Lys, and p.Trp1282\*. All other mutations are rare and many are private, only detected within a single family.

**Figure 1.** Two models for the classification of CFTR variants: (1) Functional: five classes of defective CFTR protein [3, 4, 5]; and (2) Clinical: four classes based on phenotypic expression in patients [6].

Therefore, a good knowledge of CFTR diseases and their molecular pathology is required when choosing tools and strategy and when interpreting results. Organization of regional or national networks of specialist clinical and molecular genetic laboratories is needed. Two models for the classification of sequence alterations have been proposed (Figure 1): (1) depending on their molecular impact on the CFTR chloride channel, which requires *in vitro/in vivo* functional assays, now less used due to the difficulty in accurately classifying the new reported variants [3, 4] or (2) by their clinical consequences on patients based on detailed and prospective clinical evaluation of patients associated with molecular findings [5].

However, even if the most severe mutations were studied *in vitro*, mild mutations or variants of unknown (clinical) significance (VUCs or VUS) were barely analysed. Thus, only one third of *CFTR* variations are functionally classified. Finally, in CF, as in other genetic diseases, new molecular diagnosis techniques such as Next Generation Sequencing (NGS) allow the analysis of the whole *CFTR* locus (including deep intronic regions) but increase the number of VUCS reported on patients.

Therefore, geneticists and clinicians have to combine CFTR epidemiological databases, functional *in vitro/in vivo* analysis and exhaustive clinical data to perform pertinent genotype/ phenotype correlations in patients in order to provide appropriate genetic counselling to families.
