CSK and rheumatoid arthritis: Of note, the P1 region regulates the association of PTPN22 with the Src Homology 3 (SH3) domain of C-terminal Src kinase (CSK) [76], whilst single nucleotide polymorphisms (SNPs) in this region, which disrupt PTPN22-CSK binding, have been identified as genetic risk factors for the development of autoimmune diseases, such as rheumatoid arthritis, type I diabetes and systemic lupus erythematosus [77,78,79].