We characterized thesecompounds using the well- validated Fischer rat thyroid model thathas been extensively utilized for cystic fibrosis drug discovery.9,28−30 All currently approved CFTR modulators, as well asseveral emerging compounds, have employed FRT polarized monolayersduring drug development.6,15 For specific CFTR mutationsand certain categories of pharmacologically active agents, FRT cellsexpressing mutant CFTRs have helped predict clinical benefit and contributedto regulatory approval. This evidence concerns the gene CFTR and cystic fibrosis.