Chitinase 3-like 1 (CHI3L1), also called BRP39 in mouse or YKL-40 in humans, is a 39 kDa glycoprotein member of the glycosyl hydrolase 18 family although it does not show chitotriosidase activity. CHI3L1 is expressed by articular chondrocytes, synovial cells, activated monocyte-derived macrophages, neutrophils, endothelial cells, vascular smooth muscle cells, and some cancer cells. CHI3L1 binds to chitin and heparins, and it enhances cell adhesion, proliferation and tumor angiogenesis. Circulating CHI3L1 is elevated during inflammation and connective tissue remodeling such as arthritis, chronic obstructive pulmonary disease, diabetes, cardiovascular disease, inflammatory bowel disease, and liver cirrhosis. It is frequently upregulated in glioblastoma, myxoid chondrosarcoma, melanoma and carcinomas of the breast, thyroid, colon, lung, kidney, and ovary.