Transcobalamin (TC) II deficiency is a rare autosomal recessive disorder of vitamin B12 (cobalamin, Cbl) transport that leads to intracellular Cbl depletion with secondary impairment of methionine synthetase and methyl-malonyl CoA mutase activities. This disorder presents with failure to thrive, mucosal ulceration, vomiting, diarrhoea, lethargy, irritability, and occasionally immunological dysfunction. Affected individuals may suffer from long-term neurological sequelae if therapy with intramuscular hydroxocobalamin is not initiated promptly. Mutations in the TCN2 gene are known to cause.
