PTPN22 and Autoimmunity: Intensive investigation was propelled in recent years by the discovery that a variant form of Lyp produced by a single nucleotide polymorphism (SNP) at the position 1858 (C1858T) that causes the substitution of the amino acid residue 620 from arginine (R) to tryptophan (W) in exon 14 of the Lyp protein (Lyp R620W) [2]–[6] is strongly associated with several autoimmune conditions [7]–[14].