Recent reports proposed a mechanism that IDO can induce the growth inhibition of tumour antigen-specific cytotoxic T cells by depleting the tumour microenvironment of tryptophan and/or by generating toxic tryptophan metabolites, which causes the tumour to escape from the host immune systems, leading to the uncontrolled tumour progression (Uyttenhove et al, 2003; Munn et al, 2004). This evidence concerns the gene IDO1 and neoplasm.