PTPN22 and Autoimmunity: PTPN22 interacts with CSK (Cloutier and Veillette, 1996), and this interaction is relevant for PTPN22 function, as shown by the fact that a human PTPN22C1858T variant, encoding an amino acid R620W substitution which impairs its interaction with CSK (Bottini et al., 2004), is associated with increased risk of autoimmunity (Bottini et al., 2004; Totaro et al., 2011; de Lima et al., 2017; Tizaoui et al., 2019).