MTNR1A and hepatocellular carcinoma: In term of serving as a regulator of the immune system in tumor prevention, both MT1 and MT2 are expressed in a wide variety of immune cells and contribute to inhibition of the formation of virus-induced tumors by eliminating viral infections (HCC is commonly associated with HBV and HCV infection) and identification and destruction of tumor cells that express tumor-specific antigens [44–47].