Abetalipoproteinemia is a rare, autosomal recessive disease of the lipid metabolism which is characterized by severely reduced quantities of apolipoprotein B. Already during infancy growth impairment due to a malabsorption syndrome may occur. Later in life patients show ataxia, loss of deep tendon reflexes, pigmentation disorders of the retina and acanthocytosis caused by reduced absorption of fat-soluble vitamins. Biochemical findings include decrease of total cholesterol below 50 mg/dl, decrease of apo B-containing lipoproteins and small amounts of immunoreactive Apo B in blood serum. The molecular cause for the disease are mutations in the microsomal transfer protein (MTP), a multifunctional heterodimeric protein which is involved in assembly and secretion of apo B-containing lipoproteins in the liver - mostly LDL and VLDL. The impaired function of MTP further leads to an increasing degradation of Apo B in the liver. As synthesis and secretion of Apo B-containing chylomicrons in the intestine are affected as well, a malabsorption of fat-soluble vitamins occurs. Early dietary measures along with substitution of vitamin A and E may halt progression of neurological symptoms and retinopathies.