PTPN22 and type 1 diabetes mellitus: A single-nucleotide polymorphism (SNP) in protein tyrosine phosphatase nonreceptor type 22 (PTPN22) (W620) that negatively regulates BCR downstream signaling is associated with increased risks of several autoimmune diseases, including SLE, RA, and type 1 diabetes (T1D), suggesting a fundamental role for BCR signaling in the development of autoimmunity [130–132].