CFTR and cystic fibrosis: However, patients with the same CFTR genotype may display a clinical discordance3 and a percentage of sibling pairs affected by CF display a discordant clinical manifestation3,4 reinforcing the concept that complex alleles (i.e., additional mutations on the same allele5), non-coding regions of CFTR6,7 or other genes, inherited independently of CFTR, modulate the clinical manifestation and complications of patients with CF8.