Nicotinamide adenine dinucleotide (NAD+) is essential for life in all organisms, both as a coenzyme for oxidoreductases and as a source of ADP-ribosyl groups used in various reactions. Nicotinic acid and nicotinamide, collectively known as niacin, are the vitamin precursors of NAD+. Nicotinamide riboside kinases, such as NRK1, function to synthesize NAD+ through nicotinamide mononucleotide using nicotinamide riboside as the precursor (Bieganowski and Brenner, 2004 [PubMed 15137942]).[supplied by OMIM, Mar 2008]