CFTR and cystic fibrosis: Regarding Cystic fibrosis, the 5′ss mutations in CFTR exon 12 showed a variable response to ExSpeU1, in this case, the rescue efficiency was optimal (~85%) for the +3G, intermediate (~50%) for the −1A, +3C and +5A and low (~20%) for the −1T (see mutations in Table 1).