Tezacaftor, also known as VX-661, is CFTR modulator. VX-661 is potentially useful for treatment of cystic fibrosis disease. Cystic fibrosis (CF) is a genetic disease caused by defects in the CF transmembrane regulator (CFTR) gene, which encodes an epithelial chloride channel. The most common mutation, 508CFTR, produces a protein that is misfolded and does not reach the cell membrane. VX-661 can correct trafficking of 508CFTR and partially restore chloride channel activity.