In the context of this cancer type, it has been reported that miR-126-5p promotes tumor cell proliferation, metastasis, and invasion by targeting tryptophan 2,3-dioxygenase (TDO2), which is a key enzyme in the tryptophan–kynurenine metabolic pathway (Figure 2, right bottom) [58]. Here, TDO2 is linked to neoplasm.