PRG4 (Proteoglycan 4): PRG4 is a protein, also called lubricin, that plays a crucial role in joint health by maintaining lubrication and cartilage integrity. It helps reduce friction in the joints and protects cartilage surfaces from wear and damage. Mutations or deficiencies in PRG4 can lead to joint disorders such as camptodactyly-arthropathy-coxa vara-pericarditis (CACP) syndrome and are linked to the development of osteoarthritis.