In clinical trials, IDO1 inhibitors such as epacadostat were combined with immune checkpoint blockade, such as anti-programmed death-ligand 1 (anti–PD-L1) or anti–programmed cell death protein 1 (anti-PD-1; refs. 17, 18); however, no significant improvement in tumor response was observed (19). The gene discussed is IDO1; the disease is neoplasm.