PTPN22 and type 1 diabetes mellitus: The 1858C>T allele of the tyrosine phosphatase PTPN22 (causing amino acid substitution R620W) is present in 5–10% of the North American population and is strongly associated with numerous autoimmune diseases, including Type I Diabetes (T1D), rheumatoid arthritis, systemic lupus erythematosus, and others [2–10].