PDCD1 and cancer: Three decades of intensive efforts to comprehensively characterize this new member of the immunoglobulin gene superfamily has resulted in (i) the discovery of PD-L1 [206,207] and PD-L2 [208,209], two PD-1 ligands that are widely expressed across tissues and cell populations [86,88,101], (ii) the characterization of PD-1 inhibitory function on T cells in chronic infection and cancer [102,103,210], (iii) better understanding of the role of PD-1 in autoimmunity [91,92], and (iv) the appraisal of widespread PD-1 expression across many immune cell types [101,162,211,212].