Indoleamine 2,3 dioxygenase-1 (IDO1) is a target of cancer immunotherapy. Epacadostat (INCB24360) is an oral IDO1 inhibitor. IDO1 activation blocks T-cell activation, enhances T-cell apoptosis and promotes the differentiation of naïve T cells to regulatory T cells (Tregs). Preclinical findings suggest that IDO1 is critical for cancers to evade immune surveillance and can be exploited for cancer immunotherapy.