For example, ferret tracheal submucosal gland mucus contains the proteases cathepsin S and H and their antiprotease cystatin C, and in CF ferrets the two proteases were increased more than the antiprotease, leading to an imbalance that inhibited ENaC–mediated fluid absorption and raised the height of the airway surface liquid (ASL) yet decreased MCCV64. This evidence concerns the gene CST3 and cystic fibrosis.