CFTR and cystic fibrosis: The increased efficacy observed in the presence of a gradient may be due to several factors, including minimal CFTR activity in uncorrected CF epithelia or CFTR‐mediated involvement in other mechanisms of chloride transport (Castellani et al., 2012; Egan et al., 1992; Fulmer et al., 1995; Gabriel et al., 1993; LeSimple et al., 2010; Ruan et al., 2014; Sato et al., 2019).