PTPN22 and systemic lupus erythematosus: Apart from the HLA system, genes outside the HLA system, such as PTPN22, BLK, BANK1, PXK, TNFSF4, ETS1, IKZF1, IKZF2, IKZF3, IL10, and BAFF, also play roles in T- and B-cell signaling, transcription factors, and cytokines, and have been implicated in SLE susceptibility [27,63].