MTR and cancer: Importantly, such functional B12 deficiency would be expected to be tolerated in cancer cells in met+ conditions (including in vivo), since the only human enzymes that require B12 are MTR and MUT, of which MUT is only necessary for propanoate oxidation and MTR appears to be dispensable for growth, except for a small amount of activity required to recover folates from CH3-THF [19, 41].