IDO1 and psoriasis: Finally, the genes coding for NAD+ biosynthetic enzymes involved in de novo pathway were also altered: While the transcript levels of IDO1, encoding indoleamine 2,3-dioxygenase 1, and TDO2, encoding tryptophan 2,3-dioxygenase, were induced in lesional skin, QPRT, encoding quinolinate phosphoribosyltransferase, slightly decreased in psoriasis compared to healthy skin (S7A Fig).