In humans [14, 15] and dogs [16–20] various gene variants in both CUBN and AMN are found causing hereditary selective intestinal cobalamin malabsorption with mid- to low-molecular weight proteinuria, an autosomal recessive trait also known as Imerslund-Gräsbeck syndrome (I-GS) [21, 22]. The gene discussed is CUBN; the disease is Gräsbeck-Imerslund disease.