Several mutations have been identified in the first nucleotide binding fold (NBF) of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) gene. We have analyzed the DNA sequences of exons 10 and 11 in five different mammalian species, marmoset, mouse, cow, pig, and sheep; the amino acid conservation studied for nine disease mutations; and two "benign" mutations. For exon 10, 87% homology at the DNA level and 93.5% at the amino acid level were found for these species. For exon 11, the lowest homology (70%) was found in mouse and the highest in marmoset (93%), whereas the amino acid sequence conservation ranged from 82.5 to 100%. All codons involved in CF mutations are highly conserved throughout evolution.