A major challenge for cancer vaccines is to overcome the numerous obstacles to tumor destruction, including tumor heterogeneity, immune escape mechanisms such as loss of HLA-class I molecules or T-cell inactivation by inhibitory immune checkpoints, suppressive cells (regulatory T cells, myeloid-derived suppressor cells), anti-inflammatory cytokines (IL-10, TGF-β) or enzymes (indoleamine 2,3-dioxygenase) present within the TME. The gene discussed is IDO2; the disease is cancer.