PHGDH and neoplasm: Third, using 13C-glucose tracing, we also discovered a role for PHGDH in regulating glucose carbon flux through the PPP, thereby indirectly affecting nucleotide synthesis via decreased generation of ribose-5-phosphate.26 Together, these observations suggest a mechanism by which the SSP is coupled to—and thus controls—several metabolic pathways that fuel chondrocyte proliferation, similar to observations in certain tumor cell lines.27