In many cancer types as well as tumor-infiltrating myeloid cells IDO1/kynurenine is increased [96, 97], which supports cancer cell survival: Tryptophan-derived substrates support de-novo NAD+ synthesis in tumor cells and IDO1 restricts tryptophan availability to immune cells in the tumor microenvironment (TME). The gene discussed is IDO1; the disease is neoplasm.