Gastric intrinsic factor (GIF) deficiency, Imerslund-Graesbeck syndrome (AMN; CUBN) and transcobalamin deficiency (TCN2) are inborn disorders of Cbl absorption; and the cblF (LMBRD1), cblJ (ABCD4), cblC (MMACHC) and cblD-MMAHcy (MMADHC) defects impair the shared intracellular processing pathway of Cbl. Here, TCN2 is linked to hyperinsulinemic hypoglycemia, familial, 4.