Another predictor of CF status, L-methionine-S-oxide, is a byproduct of the oxidation of methionine by myeloperoxidase in periods of oxidative stress (Pattison and Davies, 2001; Denkel et al., 2011) and is correlated with structural lung damage and airway neutrophils in early stages of CF (Chandler et al., 2018). The gene discussed is MPO; the disease is cystic fibrosis.