Full length OPN (OPN-FL) is expressed at similar levels in the synovial fluid of patients with rheumatoid arthritis (RA) and in those with osteoarthritis, but OPN-N levels in RA synovial fluid samples are around 30-fold higher than in those from osteoarthritis and correlate with the disease status [29]. The gene discussed is SPP1; the disease is rheumatoid arthritis.