We hypothesized that Slc6a14 knock-out would worsen the CF phenotype, hence, we chose the Cftr(F508del/F508del) mouse, using the FVB strain, as it exhibits a relatively mild CF intestinal phenotype due to residual F508del-CFTR function and would permit quantification of a deleterious effect (van Doorninck et al., 1995). The gene discussed is CFTR; the disease is cystic fibrosis.