Vitamin B12 (cobalamin) is essential in animals and humans for metabolism of methylmalonic acid, for the remethylation of homocysteine to methionine and, consequently, for all S-adenosylmethionine-dependent methylation reactions, including DNA synthesis. The lysosomal cobalamin transporter is required for the export cobalamin from lysosomes allowing its conversion to cofactors. Defects in LMBRD1 are the cause of methylmalonic aciduria and homocystinuria type cblF (MMAFHC), also known as homocystinuria-megaloblastic anemia complementation type F. MMAFHC is a disorder of cobalamin metabolism characterized by decreased levels of the coenzymes adenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl) due to accumulation of cobalamin in lysosomes. Clinical features of MMAFHC include developmental delay, stomatitis, glossitis, seizures and methylmalonic aciduria in response to vitamin B12. LMBRD2 (LMBR1 domain containing 2) is a 695 amino acid multi-membrane protein that may have similar functions as LMBR1.