Another substance in the synovial fluid is a human protein encoded by PRG4 gene called lubricin or proteoglycan 4 (PRC4), it is the primary lubricant of the cartilage. Lubricin is present in synovial fluid and on the surface (superficial layer) of articular cartilage; it plays a crucial role in the maintenance of the slide facility of the joint and the regulation/homeostasis of the synovial fluid.

The expression of lubricin has been detected and the protein localized in tendons, meniscus, musculature, skin, in lungs, heart and skin and can be built by mesenchymal stem cells. Lubricin, hyaluronic acid and aggrecan are the three main components of the mechanical joint activity.