Transcobalamin II deficiency is a rare autosomal recessive disorder causing intracellular cobalamin depletion, which in turn causes megaloblastic bone marrow failure, accumulation of homocysteine and methylmalonic acid with clinical findings of failure to thrive, diarrhea, vomiting, pancytopenia, megaloblastic anemia, and neurological findings [2]. Here, TCN2 is linked to hyperinsulinemic hypoglycemia, familial, 4.