Nicotinamide mononucleotide adenylyltransferase 1 (NMNAT1) (EC 2.7.7.1) is a central enzyme in NAD biosynthesis, transferring the adenylyl moiety of ATP to nicotinamide mononucleotide (NMN) or nicotinic acid mononucleotide (NaMN) resulting in the formation of NAD or NaAD and the release of pyrophosphate. As this reaction is reversible, the enzyme may in principle be used to form ATP and NMN from NAD and pyrophosphate. This enzyme could be a potential target for therapeutical applications, because its activity is rather low in tumor cells. The deduced protein contains an N-terminal nuclear localization signal. Immunofluorescence microscopy localized endogenous NMNAT1 to the nucleus in human fibroblasts and in a hepatoma cell line. It was demonstrated that NMNAT1 inhibited recombinant human poly(ADP-ribose) polymerase-1 by about 35%, and it completely prevented the formation of branched ADP-ribose polymers.