Nicotinamide adenine dinucleotide (NAD+) is an essential cofactor involved in fundamental processes in cell metabolism. NRK1 (Nicotinamide riboside kinase 1), also known as Ribosylnicotinamide kinase 1, is a 199 amino acid enzyme is involved in the synthesis of NAD+ through nicotinamide mononucleotide using nicotinamide riboside as the precursor. Nicotinamide riboside has been idenitifed as a nutrient in milk, suggesting that it is a useful compound for elevating the NAD+ levels in humans. NRK1 also phosphorylates the anti-cancer drugs tiazofurin and 3-deazaguanosine, which converts them into toxic NAD+ analogs and leads to the inhibition of guanine nucleotide biosynthesis. There are two isoforms of NRK1 that are produced as a result of alternative splicing events.