An alternative method of unleashing anti-tumor immune response is to target the indoleamine 2,3-dioxygenase (IDO) pathway (Liu et al., 2010; Munn, 2012), a metabolic pathway that suppresses immune response (Frumento et al., 2002; Terness et al., 2002; Wang et al., 2014; Holmgaard et al., 2015). The gene discussed is IDO1; the disease is neoplasm.