Using a new in vivo screening system, researchers have identified a protein in a key metabolic pathway that is essential in estrogen receptor (ER)-negative breast cancer. When the expression of the gene that codes for this protein — phosphoglycerate dehydrogenase or PHGDH — is suppressed in tumors and cell lines with an overabundance of the protein, the rate of cellular growth declines markedly. PHGDH is overexpressed in approximately 70 percent of ER-negative breast cancer patients.