PRG4 and arthropathy: Proteoglycan 4 (PRG4), which is homologous to lubricin [1], superficial zone protein (SZP) [2], megakaryocyte-stimulating factor precursor [2] and camptodactyly-arthropathy-coxavara-pericarditis protein [3], is a lubricating glycoprotein believed to be primarily responsible for boundary lubrication in synovial joints [4].