Patients with the TT genotype (two copies of the CFTR R117C 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 R117C. Other genetic and clinical factors may also influence response to ivacaftor.