CFTR and cystic fibrosis: The prevailing CF drug discovery paradigm suggests that drugs that are effective in targeting ΔF508‐CFTR will be effective in rescuing other Class 2 CFTR mutants (Awatade et al, 2015; Rapino et al, 2015). However, in testing this hypothesis for a rare CF‐causing mutation: c.3700 A>G (p.Ile1234_Arg1239del‐CFTR or ΔI1234_R1239‐CFTR; Molinski et al, 2014), we found that in primary nasal epithelial cultures, the combination therapy of lumacaftor and ivacaftor failed to rescue the mutant's functional expression to a degree considered to be therapeutically relevant.