Data from the literature indicate that in many pathologic states (cerebral ischemia, Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, multiple sclerosis, amyotrophic lateral sclerosis, autoimmune diseases, and tumor), IDO1/2 activity increases in microglial/macrophage cells (Munn, 1998; Widner et al., 1999; Tikka et al., 2001; Muller et al., 2005; Stoy et al., 2005; Munn and Mellor, 2007; Darlington et al., 2007; Opitz et al., 2007; Zelante et al., 2009; Laugeray et al., 2010; Sage et al., 2014; Mazarei and Leavitt, 2015). The gene discussed is IDO1; the disease is Parkinson disease.