Serine hydroxymethyltransferase (SHMT) is the key enzyme involved in serine catabolism.[5] SHMT has two isoforms in the human body: SHMT1 is located in the cytoplasm, and SHMT2 is located in the mitochondria.[6] There is also a short form of SHMT2, SHMT2α, located in the cytoplasm; this protein mainly functions in the BRISC complex and has no enzymatic activity toward serine.[7, 8, 9] The functions of SHMT1 and SHMT2 may be different in cancer cells. This evidence concerns the gene SHMT1 and cancer.