Because 85% of adult CF patients are chronically colonized by P. aeruginosa, and because P. aeruginosa inhibited VX-809 and VX-809 +VX-770 F508del-CFTR Cl secretion by CFBE and CF-HBE cells, the observation in this paper may explain, in part, why VX-809 + VX-770 has only modest clinical benefit. The gene discussed is CFTR; the disease is cystic fibrosis.