IDO1 and neoplasm: The relationship between IDO1-expressing tumours and T lymphocytes is, however, complex, as interferon-γ (IFN-γ), produced mainly by T lymphocytes in response to various immune stimuli (Farrar and Schreiber, 1993), is a major inducer of IDO1 (Takikawa et al, 1999; Hwu et al, 2000).