Patients with the rs397508442 TT genotype (two copies of the CFTR S945L variant) and cystic fibrosis may respond to ivacaftor treatment. FDA-approved drug labeling information and CPIC guidelines indicate use of ivacaftor in cystic fibrosis patients with at least one copy of a list of 33 CFTR genetic variants, including S945L. Other genetic and clinical factors may also influence response to ivacaftor.