CFTR and cystic fibrosis: Detailed analysis of CFTR gene and CFTR mRNA obtained from nasal cultures of a CF patient homozygous for c.3700A>G, showed that this mutation activates a cryptic donor splice site (with no wild-type (WT) transcript), resulting in deletion of six amino acids (I1234-R1239del) in NBD2, which like the F508del mutation, was shown to cause a primary defect in folding and processing [14].