IDO not only plays a key role in suppressing the anti-tumor immune response in the body (Prendergast et al., 2014), but is also involved in many neurological diseases such as atherosclerosis (Cole et al., 2015), Huntington’s disease (Perkins and Stone, 1983), Alzheimer’s disease, multiple sclerosis (Fiala et al., 1998; Guillemin et al., 2003), and psychiatric disorders (Savitz et al., 2015; Erhardt et al., 2017). Here, IDO1 is linked to Alzheimer disease.