A key finding of this study is that phosphoglycerate dehydrogenase (PHGDH) redirects glycolytic flux into serine and glycine biosynthesis, contributing to oncogenesis. PHGDH is frequently amplified in human cancers, particularly in melanoma, and its expression is associated with increased serine metabolism and cell proliferation. The study shows that PHGDH amplification leads to altered metabolic flux, which may be selected during tumor development. PHGDH activity is linked to the diversion of glucose metabolism into serine biosynthesis, which supports cell growth and transformation. The study also demonstrates that PHGDH expression affects cell morphology, polarity, and viability, and that its loss impairs cell proliferation. These findings highlight the importance of metabolic reprogramming in cancer development and suggest that PHGDH could be a therapeutic target in certain cancers. The study used a combination of mass spectrometry, NMR, and stable isotope labeling to investigate glucose metabolism in cancer cells and found that a significant portion of glycolytic carbon is diverted into serine and glycine biosynthesis. The study also shows that PHGDH amplification is associated with increased serine pathway flux and that this may contribute to the pathogenesis of human cancer. The results suggest that altered metabolic flux from glucose into specific alternative pathways can be selected during tumor development and may contribute to the pathogenesis of human cancer. The study provides evidence that PHGDH is a key metabolic enzyme in cancer and that its activity is important for cell proliferation and transformation. The findings have implications for understanding the metabolic basis of cancer and for developing new therapeutic strategies targeting metabolic pathways in cancer.A key finding of this study is that phosphoglycerate dehydrogenase (PHGDH) redirects glycolytic flux into serine and glycine biosynthesis, contributing to oncogenesis. PHGDH is frequently amplified in human cancers, particularly in melanoma, and its expression is associated with increased serine metabolism and cell proliferation. The study shows that PHGDH amplification leads to altered metabolic flux, which may be selected during tumor development. PHGDH activity is linked to the diversion of glucose metabolism into serine biosynthesis, which supports cell growth and transformation. The study also demonstrates that PHGDH expression affects cell morphology, polarity, and viability, and that its loss impairs cell proliferation. These findings highlight the importance of metabolic reprogramming in cancer development and suggest that PHGDH could be a therapeutic target in certain cancers. The study used a combination of mass spectrometry, NMR, and stable isotope labeling to investigate glucose metabolism in cancer cells and found that a significant portion of glycolytic carbon is diverted into serine and glycine biosynthesis. The study also shows that PHGDH amplification is associated with increased serine pathway flux and that this may contribute to the pathogenesis of human cancer. The results suggest that altered metabolic flux from glucose into specific alternative pathways can be selected during tumor development and may contribute to the pathogenesis of human cancer. The study provides evidence that PHGDH is a key metabolic enzyme in cancer and that its activity is important for cell proliferation and transformation. The findings have implications for understanding the metabolic basis of cancer and for developing new therapeutic strategies targeting metabolic pathways in cancer.