CFTR and cystic fibrosis: Likewise, Bartoszewski et al. (2010) showed that the trinucleotide deletion causing F508del, which is found in the majority of patients with CF (i.e., the out-of-frame CTT deletion between amino acids Ile507 and Phe508) and rendering a synonymous single nucleotide polymorphism at Ile507, caused instability of F508del-CFTR mRNA due to the enhanced size of hairpin loops relative to wild-type CFTR mRNA.