PDCD1 and rheumatoid arthritis: Studies on the PD-1 gene have demonstrated that the polymorphisms of this gene are associated with several autoimmune diseases including systemic lupus erythematosus (SLE) [16]–[20], rheumatoid arthritis (RA) [21], type I diabetes [22], [23], multiple sclerosis [24], ankylosing spondylitis (AS) [25], and Graves' disease (GD) [26], although there are also some conflicting results [27]–[32].