PDCD1 and systemic lupus erythematosus: The observations from these animal models are recapitulated in a human population with regulatory polymorphisms in the PDCD1 gene, which codes for PD-1, which has increased incidence of systemic lupus erythematosus (SLE) (Prokunina et al., 2002), atopy, and rheumatoid arthritis (RA) (James et al., 2005; Lee et al., 2015b) and progression in multiple sclerosis (MS) (Kroner et al., 2005).