Patients with the TT genotype (do not have a copy of the CFTR L206W variant) and cystic fibrosis have an unknown response to ivacaftor treatment, as response may depend on the presence of other CFTR variants. 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 L206W. Other genetic and clinical factors may also influence response to ivacaftor.