CFTR and cystic fibrosis: A phase II trial is in progress to comparatively evaluate the effects of VX-561 and ivacaftor in CF patients who have the following CFTR gating mutations in at least one allele: G178R, S549N, S549R, G551D, G551S, G1244E, S1251N, S1255P, or G1349D (NCT03911713).