Serum YKL-40 is positively correlated to low-attenuation area percentage a marker of the extent of lung emphysema, and negatively correlated to forced expiratory volume in 1 second (FEV1)% predicted, a marker of disease severity, in patients with COPD[28] and patients with asthma[8]. This evidence concerns the gene CHI3L1 and chronic obstructive pulmonary disease.