This study investigates the role of phosphoglycerate dehydrogenase (PHGDH) in cancer metabolism and oncogenesis. Using mass spectrometry and NMR with stable isotope labeling, the authors found that a significant portion of glycolytic carbon is diverted into serine and glycine biosynthesis through PHGDH in some cancer cells. Bioinformatics analysis of 3131 human cancers revealed recurrent amplification of the *PHGDH* gene at 1p12, which is associated with increased protein expression and is commonly found in melanoma. Knockdown of *PHGDH* impaired growth and flux into serine metabolism in *PHGDH*-amplified cell lines. Increased *PHGDH* expression was also associated with breast cancer subtypes and ectopic expression in mammary epithelial cells (MCF-10a) disrupted acinar morphogenesis, induced loss of polarity, and preserved the viability of extracellular matrix-deprived cells. These findings demonstrate that altered metabolic flux from glucose into specific alternate pathways can be selected during tumor development and may contribute to human cancer pathogenesis.This study investigates the role of phosphoglycerate dehydrogenase (PHGDH) in cancer metabolism and oncogenesis. Using mass spectrometry and NMR with stable isotope labeling, the authors found that a significant portion of glycolytic carbon is diverted into serine and glycine biosynthesis through PHGDH in some cancer cells. Bioinformatics analysis of 3131 human cancers revealed recurrent amplification of the *PHGDH* gene at 1p12, which is associated with increased protein expression and is commonly found in melanoma. Knockdown of *PHGDH* impaired growth and flux into serine metabolism in *PHGDH*-amplified cell lines. Increased *PHGDH* expression was also associated with breast cancer subtypes and ectopic expression in mammary epithelial cells (MCF-10a) disrupted acinar morphogenesis, induced loss of polarity, and preserved the viability of extracellular matrix-deprived cells. These findings demonstrate that altered metabolic flux from glucose into specific alternate pathways can be selected during tumor development and may contribute to human cancer pathogenesis.