Chondrocyte protein 39 (YKL-39), also known as Chitinase 3-like 2 (CHI3L2), is a secretory protein of articular chondrocytes belonging to the glycosyl hydrolase 18 family. Its highest expression is in chondrocytes, followed by synoviocytes, lung and heart. YKL-39/CHI3L2 is not detected in spleen, pancreas, and liver. YKL-39/CHI3L2 may also be expressed in developing brain and placenta. YKL-39/CHI3L2, a cartilage-related protein, is found to induce arthritis accompanied by pathologic changes in bone and cartilage. A better understanding of the immune response against cartilage-related components including YKL-39 may help to elucidate the pathological processes of arthritic disorders. Upregulation of YKL-39/CHI3L2 in osteoarthritic cartilage suggests that YKL-39/CHI3L2 may be a more accurate marker of chondrocyte activation than YKL-40, although it has yet to be established as a suitable marker in synovial fluid and serum. The decreased expression of YKL-40 by osteoarthritic chondrocytes is surprising as increased levels have been reported in rheumatoid and osteoarthritic synovial fluid, where it may derive from activated synovial cells or osteophytic tissue or by increased matrix destruction in the osteoarthritic joint. YKL-39 and YKL-40 are potentially interesting marker molecules for arthritic joint disease because they are abundantly expressed by both normal and osteoarthritic chondrocytes.