SPP1 and rheumatoid arthritis: Osteopontin (OPN) is a pro-inflammatory cytokine, and increased circulating levels have been associated with inflammatory muscle diseases and muscular dystrophy (10–13) as well as the onset and progression of Crohn’s disease, myocarditis, uveitis, idiopathic retroperitoneal fibrosis, and rheumatoid arthritis (14–16).