CFTR and cystic fibrosis: To test whether naturally occurring CF-causing nonsense variants in exon 22 encoding ICL6 (codons 1150 to 1218) allow production of stable and potentially drug-targetable forms of truncated CFTR, we utilized EMG-i21-i22 that contains abridged introns 21 and 22 incorporated into full-length CFTR cDNA to evaluate variant effects (Fig 2A, top, S3 Fig).