Defects in ApoB synthesis and secretion, resulting from mutations in the ApoB, microsomal triglyceride transfer protein (MTP), and proprotein convertase subtilisin/kexin type 9 genes, lead to abetalipoproteinemia (ABL) and familial hypobetalipoproteinemia (FHBL; refs. 4, 5), two disorders characterized by low or absent levels of ApoB and LDL-cholesterol in plasma. The gene discussed is MTTP; the disease is hypobetalipoproteinemia.