CHI3L1 and pulmonary fibrosis: Since excellent review articles detailing the molecules and signaling pathways involved in the pathogenesis of pulmonary fibrosis are already available (Micallef et al., 2012; Wolters et al., 2014; Sgalla et al., 2018; Strykowski and Adegunsoye, 2021), in this section, we only focus on the discussion of major factors leading to pathologic fibrosis, that can be also regulated by CHIT1 or CHI3L1 in the development and progression of pulmonary fibrosis.