Transcobalamin II (TCII, the product of the gene TCN2) is a vitamin B12-binding protein secreted by endothelial cells into plasma that facilitates the endocytosis of cobalamin (Cbl, vitamin B12) into hepatocytes or cells requiring Cbl. Defective TCN2 results in TCN2 deficiency (MIM:275350), an autosomal recessive disorder with early-onset in infancy characterized by failure to thrive, megaloblastic anemia, and pancytopenia. If left untreated, the disorder can result in mental retardation and neurologic abnormalities (Haberle et al. 2009). In white populations, the most common mutation is pro259arg (P259R). Originally reported as a G-to-C transversion at base position 775 (Miller et al. 2002), it is actually a C-to-G substitution at position 776 (Zetterberg et al. 2003).