This project contains all information pertaining to the replication of key experiments from this paper. It includes the detailed protocols, including reagents and author clarifications. We also include any comments from other contributors, researchers from the Science Exchange network, and further information with the original authors that we have learned since the beginning of the project. When experimental studies begin all data collected will also be deposited here, including data analysis and eventually the final written report. <br> **Original citation:** Possemato R., Marks K.M., Shaul Y.D., Pacold M.E., Kim D., Birsoy K., Sethumadhavan S., Woo H.-K., Jang H.G., Jha A.K., Chen W.W., Barrett F.G., Stransky N., Tsun Z.-Y., Cowley G.S., Barretina J., Kalaany N.Y., Hsu P.P., Ottina K., Chan A.M., Yuan B., Garraway L.A., Root D.E., Mino-Kenudson M., Brachtel E.F., Driggers E.M., Sabatini D.M. Functional genomics reveal that the serine synthesis pathway is essential in breast cancer. Nature. 2011 Aug 18;476(7360):346-50. doi: 10.1038/nature10350. <br> **Original paper abstract:** Cancer cells adapt their metabolic processes to drive macromolecular biosynthesis for rapid cell growth and proliferation. RNA interference (RNAi)-based loss-of-function screening has proven powerful for the identification of new and interesting cancer targets, and recent studies have used this technology in vivo to identify novel tumour suppressor genes. Here we developed a method for identifying novel cancer targets via negative-selection RNAi screening using a human breast cancer xenograft model at an orthotopic site in the mouse. Using this method, we screened a set of metabolic genes associated with aggressive breast cancer and stemness to identify those required for in vivo tumorigenesis. Among the genes identified, phosphoglycerate dehydrogenase (PHGDH) is in a genomic region of recurrent copy number gain in breast cancer and PHGDH protein levels are elevated in 70% of oestrogen receptor (ER)-negative breast cancers. PHGDH catalyses the first step in the serine biosynthesis pathway, and breast cancer cells with high PHGDH expression have increased serine synthesis flux. Suppression of PHGDH in cell lines with elevated PHGDH expression, but not in those without, causes a strong decrease in cell proliferation and a reduction in serine synthesis. We find that PHGDH suppression does not affect intracellular serine levels, but causes a drop in the levels of alpha-ketoglutarate, another output of the pathway and a tricarboxylic acid (TCA) cycle intermediate. In cells with high PHGDH expression, the serine synthesis pathway contributes approximately 50% of the total anaplerotic flux of glutamine into the TCA cycle. These results reveal that certain breast cancers are dependent on increased serine pathway flux caused by PHGDH overexpression and demonstrate the utility of in vivo negative-selection RNAi screens for finding potential anticancer targets.