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). Here, SPP1 is linked to IgG4-related retroperitoneal fibrosis.