PKM and cancer: In cancer cells, biosynthesis of serine (SSP—Serine Biosynthesis Pathway) depends on several interconnected factors: (a) availability of the precursor, 3-phosphogylcerate (glycerate 3-phosphate; labeled green on Figure 1, green circle) whose generation is dependent on glucose intake and presence of metabolically inert pyruvate kinase 2 (PKM2), (b) ATF4-directed expression of three metabolic enzymes, (1) PHGDH—Phosphoglycerate-3-Dehydrogenase; (2) PSAT1-Phosphoserine Aminotransferase 1; and (3) PSPH-Phosphoserine Phosphatase.