Human patients with camptodactyly-arthropathy–coxa vara–pericarditis (CACP) syndrome who lack functional lubricin and Prg4 knockout mice develop severe, early-onset polyarthropathy, demonstrating the significance of lubricin in chondroprotection and joint homeostasis11–13. Here, PRG4 is linked to camptodactyly-arthropathy-coxa vara-pericarditis syndrome.