The NAD biosynthesis pathway. NAMPT can transfer a phosphoribosyl group obtained from PRPP to nicotinamide, resulting in the production of NMN and PPi. Thereafter, NMN is converted to NAD by NMNAT. NAD, nicotinamide adenine dinucleotide; NAMPT, nicotinamide phosphoribosyltransferase; PRPP, 5-phosphoribosyl-1-pyrophosphate; NMN, nicotinamide mononucleotide; PPi, pyrophosphate; NMNAT, nicotinamide mononucleotide adenylyltransferase.