The deficiency of even a single gene in this important pathway can result in severe GI disease, as occurs with glucose-galactose malabsorption syndrome caused by SGLT1 deficiency, Fanconi-Bickel syndrome resulting from GLUT2 mutations, sucrase-isomaltase deficiency, and congenital lactase deficiency [40], [41]. The gene discussed is SI; the disease is malabsorption syndrome.