CFTR and cystic fibrosis: The N1303K allele is a product of a missense mutation in exon 21 of the CFTR gene [3], causing an asparagine to lysine amino acid change in the nucleotide-binding domain 2 (NBD2) of the CFTR protein (Cystic Fibrosis Mutation Database, http://www.genet.sickkids.on.ca/cftr).