However, increased PDCD1 expression was observed in early RA (fold change (FC) = 2.52, P = 4.38E-04) and established RA (FC = 1.49, P = 2.63E-02) synovial tissue samples, but not in arthralgia or IA/UA (Fig 1A). Here, PDCD1 is linked to rheumatoid arthritis.