For example, in vitro studies have shown that the expression of tryptophan-2,3-dioxygenase in glioblastoma or breast cancer cells promotes tumor cell migration and invasion and overexpression of the rate-limiting enzyme indoleamine-2,3-dioxygenase 1 in lung cancer cells enhances the motility of tumor cells, whereas knockdown reduces their motility [34]. Here, IDO1 is linked to breast cancer.