CHI3L1 and interstitial lung disease: Our finding of increased serum YKL-40 concentrations in IPF compared with controls is in complete agreement with the previous clinical research studies in the ILD field reporting elevated YKL-40 concentrations in serum and/or BALf of patients with IPF (27–30), sarcoidosis (44, 45), or connective tissue disease-associated ILD (CTD-ILD) (29, 46–49).