Chitinase-3-like protein 1 (CHI3L1), also known as YKL-40, is a secreted glycoprotein that is approximately 40kDa in size that in humans is encoded by the CHI3L1 gene. YKL-40 is secreted by various cell-types including macrophages, chondrocytes, and some types of cancer cells. YKL-40 lacks chitinase activity due to mutations within the active site. The exact physiological role of YKL-40 is not known, but it has been implicated in development, inflammatory disease.

Chitinases catalyze the hydrolysis of chitin, which is an abundant glycopolymer found in insect exoskeletons and fungal cell walls. The protein lacks chitinase activity and is secreted by activated macrophages, chondrocytes, neutrophils and synovial cells. The protein is thought to play a role in the process of inflammation and tissue remodeling.