PYCR1 and breast cancer: Consistently, in cancer cells MYC promotes enhanced proline biosynthesis from glutamine in part by transcriptional upregulation of pyrroline-5-carboxylate synthase and pyrroline-5-carboxylate reductase 1 (ref. 29), and pyrroline-5carboxylate reductase 1 has been found to be important for optimal growth of breast cancer cell lines10.