The activities of 3-phosphoglycerate dehydrogenase, an enzyme of serine biosynthesis, and serine hydroxymethyltransferase, serine dehydratase and serine aminotransferase, which are competing enzymes of serine utilization, were assayed in human colon carcinomas from patients and in transplantable rat sarcomas. This evidence concerns the gene PHGDH and sarcoma.