CFTR and cystic fibrosis: A review of the literature led to two observations that have suggested partial redundancy of CFTR exon 9: 1) splice mutations 1209 + 1 G > C and 1209 + 2 T > G were documented to cause CFTR exon 9 skipping and these variants were reported to manifest as a milder form of disease caused by mutations in the CFTR gene; congenital bilateral absence of vas deferens (CBAVD) (Li et al., 2012) and 2) CFTR exon 9 is in-frame and is reported to carry relatively few CF-causing mutations, when compared to other CFTR exons (Toronto 2011; Henrie et al., 2018; Molinski et al., 2018).