MCEE (methylmalonyl-CoA epimerase) is a mitochondrial matrix enzyme that catalyzes the interconversion of D- and L-methylmalonyl-CoA in propionate catabolism 1. As a member of the glyoxalase gene family, MCEE functions in primary metabolism and plays a role in the detoxification of reactive dicarbonyls, contributing to cellular homeostasis 2. The enzyme operates in a critical metabolic pathway converting propionyl-CoA to succinyl-CoA, following propionyl-CoA carboxylase and preceding methylmalonyl-CoA mutase 3. Autosomal recessive pathogenic variations in MCEE cause methylmalonic aciduria (MMAuria), characterized by elevated methylmalonic acid in blood, urine, and cerebrospinal fluid 4. Clinical presentations range from asymptomatic to acute metabolic acidosis with neurological symptoms 3. Pathogenic mechanisms include protein misfolding, dimeric assembly disruption, and loss of enzymatic activity 4. Beyond genetic disease, MCEE downregulation occurs in cancer cells through transcriptional dysregulation, leading to reduced propionate metabolism and methylmalonic acid accumulation that promotes cancer invasiveness 5. MCEE deficiency represents a rare but treatable metabolic disorder with variable clinical severity, where most patients maintain normal development on dietary management 3.