Abetalipoproteinemia (ABL; OMIM 200100) is an autosomal recessive disorder whose corebiochemical defect is impaired absorption of dietary lipids with impaired assembly andsecretion of apolipoprotein (apo) B–containing lipoprotein particles.1 Biallelic pathogenic mutations have been reported in all functional domains of theMTTP gene that encodes the microsomal triglyceride (TG) transfer proteinlarge subunit (MTP) in ABL patients.2 MTP plays a critical role in the assembly of lipoproteins. The gene discussed is MTTP; the disease is abetalipoproteinemia.