LMBRD1 encodes a lysosomal membrane protein primarily functioning as a vitamin B₁₂ (cobalamin) exporter 1. The protein contains nine transmembrane domains and shows homology to lipocalin membrane receptors 2. LMBRD1 forms homodimers and interacts with ABCD4 to facilitate vectorial delivery of lysosomal cobalamin to the cytoplasmic protein MMACHC, preventing cofactor dilution and protecting against inactivating reactions 3. Loss-of-function mutations in LMBRD1 cause cblF deficiency, a rare autosomal recessive inborn error of cobalamin metabolism characterized by lysosomal accumulation of free vitamin B₁₂ and impaired synthesis of vitamin B₁₂-dependent enzymes 21. Clinical manifestations include neurological, hematological, developmental, and dermatological defects; patients respond to hydroxocobalamin therapy 4. Heterozygous LMBRD1 knockout in mice causes cardiac hypertrophy, ventricular fibrosis, and impaired cardiac function through altered insulin receptor signaling and increased myocardial glucose uptake, independent of cobalamin transport defects 5. Additionally, LMBRD1 variants associate with methylmalonic aciduria combined with homocystinuria 6 and may contribute to cardiovascular laterality defects 7. LMBRD1 demonstrates prognostic significance in lung adenocarcinoma, where poor expression correlates with worse outcomes 8.