PTPN22 and autoimmune disease: Considering the female predominance in most of the autoimmune disorders associated with the PTPN22 Trp620 variant, Nielsen et al. investigated the existence of cis-acting or sex-specific trans-acting factor/s (e.g., sex hormones) affecting the allele-specific expression of the PTPN22 Arg620Trp polymorphism [105].