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