Through competitively binding miR-4738-3p, circZNF566 blocks the direct interaction of miR-4738-3p and the 3’-UTR of tryptophan 2,3-dioxygenase (TDO2) mRNA, which further leads to the upregulation of TDO2, thereby promoting the proliferation, invasion, and metastasis of HCC cells, while cancer-promoting effect can be reversed by the knockdown of circZNF566 (50). Here, TDO2 is linked to cancer.