L1mT is a competitive inhibitor and substrate of IDO1 enzymatic activity (Ki = 19 μM) in cell-free assays using purified recombinant IDO1 enzyme [49], and in tumor cells treated with INFγ or in tumor cell lines transfected with expression vectors that encode IDO1 under the control of an heterologous promoter [49]. This evidence concerns the gene IDO1 and neoplasm.