PSAT1 and cancer: PHGDH and the other enzymes in the serine synthesis pathway—phosphoserine aminotransferase 1 (PSAT1) and phosphoserine phosphatase (PSPH)—can also be activated in cancer cells by epigenetic mechanisms [60] and by the transcription factor ATF4 downstream of both mTOR and Nrf2 signaling [61,62].