IDO1 and neoplasm: We found that the expression and activity of IDO was impaired by hypoxia in different tumour cells (86HG39 and HeLa cells) and native cells (HFF cells) and that the tryptophan-dependent growth-inhibition of three prototypic pathogens in these cells was compromised, resulting in an unhindered proliferation of extracellular bacteria (Staphylococcus aureus), the intracellular parasite Toxoplasma gondii and herpes simplex virus type 1.