Other genes implicated in the adaptive immune system, including PTPN22, PDCD1 (encodes PD-1) [24], BANK-1 [25], BLK, LYN, and TNFRSF4 (OX40L), indicate that the threshold for activation of B and T cells is important in SLE [26,27,28,29]. Here, PDCD1 is linked to systemic lupus erythematosus.