Rare mutations in some of the B12 genes described here i.e. MMACHC, MMAA, MUT, CD320, TCN2 and CUBN have been described in connection with rare conditions of methylmalonic aciduria and megaloblastic anemia that all relate to defects in B12 metabolism (OMIM database, http://www.ncbi.nlm.nih.gov/omim/). Here, TCN2 is linked to megaloblastic anemia.