N-methylation is one method by which drug and other xenobiotic compounds are metabolized by the liver. NNMT is an important cytosolic methyltransferase catalyzing the N-methylation of nicotinamide, pyridines and structural analogs, playing a crucial role in the biotransformation and detoxification of many xenobiotic compounds. In the N-methylation process, NNMT uses S-adenosyl methionine as the methyl donor and nicotinamide as methyl acceptor. NNMT is mostly expressed in the liver, and a lower expression is seen in the kidney, lung, skeletal muscle, placenta and heart. It may also play an important role in regulating biological processes related to N-methyl nicotinamide such as anti-inflammatory, anti-thrombotic, vasoprotective, and gastroprotective properties. Increased NNMT activity was reported in many kinds of tumors. NNMT is a potential biomarker and therapeutic target in cancer diagnosis and treatment. NNMT serum levels have significance in the premature detection and in the management of patients with colorectal cancer.