*4.1. CFTR Variants, Cells, and Culture Conditions*

Several CFTR deletion variants were produced by site-directed mutagenesis corresponding to the removal of residues: ∆RIL– Gly-Leu435; ∆RIS– Ala-Leu428; ∆REL– Cys-Ser678; ∆RES– Ser-Gly673; ∆H9 –637Gln-Gly646; ∆REL-∆H9 –637Gln-Ser678; ∆RIS-∆RES–both <sup>412</sup>Ala-Leu<sup>428</sup> and Ser-Gly673; and ∆RIL-∆RES– Gly-Leu<sup>435</sup> and <sup>654</sup>Ser-Gly673. All the constructs were produced using full length wt-CFTR and F508del-CFTR.

BHK cells lines expressing ∆RIL-, ∆RIL-F508del-, ∆RIL-G550E-, ∆RIL-G550E-F508del, ∆RIL-R1070W-, ∆RIL-R1070W-F508del, ∆RIS-, ∆RIS-F508del, ∆RI<sup>S</sup> -G550E-, ∆RIS-G550E-F508del, ∆RIS-R1070W-, ∆RIS-R1070W-F508del, ∆RES-, ∆RES-F508del, ∆RES-G550E, ∆RES-G550E-F508del, ∆RES-R1070W-, ∆RES-R1070W-F508del, ∆REL-, ∆REL-F508del, ∆RIS-∆RE-, ∆RIS-∆RES-F508del, ∆RIL-∆RE-, ∆RIL- ∆RES-F508del, ∆H9-, ∆H9-F508del, ∆REL-∆H9-, and ∆REL-∆H9-F508del-CFTR were produced and cultured as previously described <sup>11</sup>. Cells were cultured in DMEM/F-12 medium containing 5% (*v*/*v*) Fetal bovine serum (FBS) and 500 µM of methotrexate. For some experiments, cells were incubated with 3 µM VX-809 or with the equivalent concentration of Dimethyl sulfoxide (DMSO, Control) for 48 h at 37 ◦C.
