PTPN22 and type 1 diabetes mellitus: With respect to peripheral tolerance, gene variants at other loci such as protein tyrosine phosphatase, non-receptor type 22 (PTPN22) may accelerate T1D onset through mechanisms that have not been fully elucidated (Pociot and McDermott, 2002; Bottini et al., 2004; Luca et al., 2008; Wallis et al., 2009).