VX-809 is the second investigational oral candidate compound for the treatment of cystic fibrosis (CF). VX-809 may act to restore the function of the cystic fibrosis transmembrane conductance regulator (CFTR) protein, the defective cell membrane protein responsible for the progression of CF. VX-809 defects in the CFTR protein affect the transport of chloride and other ions across cells, and lead to the accumulation of thick, sticky mucus in the lungs of patients with CF. This mucus fosters chronic infection and inflammation, and results in irreversible lung damage.