LC–MS-based metabolomics on tumor lysates demonstrated lower tumor serine and glycine levels upon deletion of SLC6A14/12A4 and SLC6A14/25A15 in HCT116 PHGDH-deficient xenografts, whereas serine and glycine levels in the circulating blood remained unaltered (Fig. 7h–k). The gene discussed is PHGDH; the disease is neoplasm.