Patients with the rs121908755 AA genotype (two copies of the CFTR S549N 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 S549N. Other genetic and clinical factors may also influence response to ivacaftor.