Chitinase 3-like 1 (CHI3L1), also called YKL-40 or human cartilage glycoprotein 39 (HCGP39), is a 39 kDa glycoprotein member of the glycosyl hydrolase 18 family (1‑4). CHI3L1 was first identified as secreted from cultured articular chondrocytes, synovial cells, and activated monocyte‑derived macrophages, but it is also secreted by neutrophils, endothelial cells, vascular smooth muscle cells, and some cancer cells (1, 2, 5‑7). The human CHI3L1 cDNA encodes 383 amino acids (aa), including a 21 aa signal sequence and a 362 aa mature region with two intramolecular disulfides (3). Mature human CHI3L1 shares 73%, 80%, 82%, 83%, 86% and 87% aa sequence identity with mouse, rat, canine, bovine, porcine and equine CHI3L1, respectively. Human CHI3L1 and CHI3L2 share 43% aa sequence identity. Neither shows chitotriosidase activity, but both bind chitin and are thus termed chi-lectins (1‑4). CHI3L1 can bind heparins, probably as heparan sulfate (3, 7). It has been found to enhance cell adhesion and promote cell signaling, proliferation and tumor angiogenesis (4, 6‑9). Elevated serum CHI3L1 levels occur in some conditions characterized by inflammation and connective tissue remodeling, such as arthritis, chronic obstructive pulmonary disease, diabetes, cardiovascular disease, inflammatory bowel disease, and liver cirrhosis (1, 9‑12). Single nucleotide polymorphisms that can increase serum CHI3L1 are associated with higher risk for asthma in childhood (13). CHI3L1 is frequently up‑regulated in glioblastoma, myxoid chondrosarcoma, melanoma and carcinomas of the breast, thyroid, colon, lung, kidney, and ovary (9). In asthma and cancer, serum CHI3L1 concentrations can correlate with prognosis (9, 14, 15).