Studies that have been designed to evaluate why melanomas that attract CD8+ T cells are not spontaneously rejected have pointed to several mechanisms that may exert negative regulation of T cells within the tumor microenvironment, including T cell inhibition via IDO and PD-L1, extrinsic suppression via CD4+CD25+FoxP3+ Tregs, and T cell anergy due to deficiency of B7 costimulation in the tumor microenvironment. This evidence concerns the gene IDO1 and neoplasm.