The Imerslund-Gräsbeck syndrome mutations, for which functionality was proven, affect the IF-cobalamin-binding region in the CUB8 domain of cubilin (rs121434430 Pro1297Leu), CUB6 domain (CUBN IVS6 C-G in-frame insertion) or CUB23 domain (CUBN IVS23 G-T transversion at the conserved donor splice site of exon 23). This evidence concerns the gene CUBN and Gräsbeck-Imerslund disease.