The de novo and salvage NAD biosynthetic pathways in the budding yeast Saccharomyces cerevisiae. Pnc1, Npt1, Nma1 and Nma2, Qns1, and Qpt1 are nicotinamidase, nicotinic acid phosphoribosyltransferase, nicotinic acid mononucleotide adenylyltransferase 1 and 2, NAD synthetase, and quinolinic acid phosphoribosyltransfease, respectively. This pathway is also conserved in C. elegans, Drosophila and other invertebrates. Nic, nicotinamide; NA, nicotinic acid; NaMN, nicotinic acid mononucleotide.