Mucus taken from the endotracheal tubes (ETT) of patients with no lung disease actually produced proportions of mucin similar to those found in the sputum of patients experiencing CF exacerbations (12% more MUC5AC in CF exacerbation vs. ETT, 4% less MUC5B in CF exacerbation vs. ETT) [55]. Here, MUC5AC is linked to cystic fibrosis.