SPP1 and rheumatoid arthritis: These observations were in agreement with Suzuki et al.’s findings, which found that OPN was overexpressed in the RA synovial sublining and lining layers, in the cartilage interface, and in invading synovium, in addition to the observations of OPN mRNA overexpressed in CD4+ synovial T cells that correlates with the expression of CD44 and αvβ1 integrin receptor [132,133].