YKL-39 is a 390 amino acid cartilage protein that belongs to the chitinase family of chitin-fragmenting hydrolases. Highly expressed in chondrocytes (cartilage cells) and synoviocytes (fibroblastic cells that line joint cavities), YKL-39 binds glycan structures with high affinity. Although related to bacterial chitinases, YKL-39 lacks the characteristic glutamate active site and, thus, does not have enzymatic chitinase activity. Patients affected with rheumatoid arthritis (RA) have autoimmunity against YKL-39, suggesting that YKL-39 is involved in osteoarthritic and/or rheumatoid joint disease. Additionally, YKL-39 is upregulated in early degenerative cartilage diseases (such as RA) and may be a marker of chondrocyte activation in these autoimmune conditions.