Patients with the rs397508256 AG genotype (one copy of the CFTR E56K 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 E56K. Other genetic and clinical factors may also influence response to ivacaftor.