MCEE and cancer: Aberrations in the enzymes downstream of methylmalonyl-CoA in the propionate metabolism pathway, namely methylmalonyl-CoA mutase (MUT), methylmalonyl-CoA epimerase (MCEE), methylmalonic aciduria type A protein (MMAA), or cob(I)yrinic acid a,c-diamide adenosyl-transferase (MMAB) result in pathogenic systemic MMA accumulation in methylmalonic acidemias10–13, and drive cancer drug resistance and metastasis through increased MMA accumulation in vitro and in vivo3 (Fig. 1a).