NNMT and hepatocellular carcinoma: This review systematically synthesizes current evidence on NNMT’s dual role in HCC: (1) driving NAD+ depletion and homocysteine (Hcy) accumulation via metabolic dysregulation, (2) promoting malignant phenotypes (proliferation, invasion, metastasis, and drug resistance), and (3) serving as a prognostic biomarker and therapeutic target.