Editor—Cystic fibrosis (CF, MIM 219700) is a common, severe, autosomal recessive disease caused by mutations in the CF transmembrane conductance regulator (CFTR) gene cloned in 1989.1-3The disease, characterised by chronic lung disease which is the main cause of morbidity and mortality, pancreatic dysfunction, raised electrolyte levels in sweat, and male infertility, is caused by altered chloride (Cl⁻) secretion across the apical membrane of epithelial cells.4 There is, however, substantial variability in the clinical manifestations affecting the various organs.4 5

One single mutation, F508del, generally associated with severe disease, accounts for about 70% of CF chromosomes world wide, although with a heterogeneous geographical distribution.5 Patients homozygous for the F508del mutation have the classical severe form of the disease which includes chronic mucous obstruction of the lung and conducting airways, followed by recurrent infections mostly byPseudomonas aeruginosa (Pa) andStaphylococcus aureus (Sa), exocrine pancreatic insufficiency (PI), resulting in failure to gain weight and height, and raised levels of Cl⁻, sodium, and potassium in exocrine sweat.5 However, almost 1000 genetic alterations have been detected in the CFTR gene (CFTR Mutation Database), most presumed to be disease causing mutations. About half of these are amino acid substitutions (missense mutations) and about 20% are splicing mutations. The remainder are nonsense, frameshift (including small deletions and insertions), and a small proportion of promoter mutations.

The relationship between genotype, that is, the mutations in theCFTR gene, and the clinical phenotype of CF patients has been difficult to establish, in particular for lung disease.

It was previously shown that the 3272-26A>G mutation leads to the creation of an alternative acceptor splice site competing with the normal one during RNA processing and resulting in the occurrence of an alternatively spliced mRNA with 25 extra nucleotides from …