It is plausible that the other innate host defence proteins decreased in frequent exacerbators are likewise decreased as a result of neutrophil elastase mediated degradation, since numerous innate immune proteins including A1AT, elafin, SPLUNC1 and SP-A have been reported previously to be susceptible to proteolytic cleavage in CF airway secretions [26, 36, 37, 48]. Here, PROS1 is linked to cystic fibrosis.