ABCG2 and cystic fibrosis: Cystic fibrosis (CF) is a prototypical protein folding disorder caused by mutations in the Cystic Fibrosis Transmembrane conductance Regulator (CFTR), an ABC transporter containing two six-spanning transmembrane domains (TMD1 and TMD2), two cytosolic nucleotide binding domains (NBD1 and NBD2), and an unstructured regulatory domain (R).