PTPN22 and rheumatoid arthritis: Existing evidence suggests that a single locus may contain variants that predispose to any one of multiple diseases, e.g., the nonsynonymous C1858T SNP in PTPN22 is associated with rheumatoid arthritis and T1D [Barrett et al., 2009; Stahl et al., 2010], or distinct variants that predispose to different diseases, e.g., distinct variants in IL2RA are associated with T1D and multiple sclerosis [Maier et al., 2009; Martin et al., 2012].