CHI3L1 and chronic obstructive pulmonary disease: Several candidate biomarkers have been proposed, including AGR2 [11,12], SAA [11], HER2 [11,13,14], APOE [15], or SCGB3A2 [16] for AC; YKL-40 [17], MFAP4 [18], GRP78, soluble CD163, IL1AP, and MSPT9 for COPD [11]; CRP, VEGF [13,14,19], IL-8, and MMP9 [14] for the differentiation of AC and COPD.