Updated publication reference for PubMed record(s): 30157431. Maintenance of NAD+ levels by mitochondrial complex I, the NAD+ salvage pathway, and other routes is an important factor in of neurodegenerative disease and cancer. Both the production of NAD+ and the metabolic enzymes that require it as a redox cofactor or substrate differ widely in abundance across cell types and conditions. Disruption in the NAD+ supply thus exerts different effects depending on the cellular NAD+ requirements existing in the cell. Pharmacological depletion of NAD+ is actively being pursued in cancer and other diseases but these effects are not fully understood. Here, we combine quantitative proteomics and metabolomics to understand the consequences of disrupting cellular NAD+ levels and find that inhibiting the NAD+ salvage pathway depletes serine biosynthesis from glucose by impeding the NAD+-dependent protein 3-phosphoglycerate dehydrogenase (PHGDH). Importantly, breast cancers that depend on PHGDH are exquisitely sensitive to blocking the NAD+ salvage pathway. PHGDH, and the rate-limiting enzyme of NAD+ salvage are also correlated in public tumor proteome and transcript datasets. These findings are immediately translatable to the pharmacological inhibition of NAMPT in PHGDH-dependent cancers.