Interestingly, although TDO promotes tumorigenesis in gliomas, breast cancer, melanoma, cervical cancer, and colorectal cancer, in hepatocellular carcinoma, TDO2 overexpression upregulates p21 and p27 protein levels and decreases CDK2 and CDK4 protein expression, leading to G1 phase cell cycle arrest [129]. Here, CDK4 is linked to melanoma.