Unlike homozygous embryos in abetalipoproteinemia and hypobetalipoproteinemia (with ApoB and MTTP deletions, respectively), which developed malformations leading to embryo resorption (27, 28), homozygous Sar1bdel/del and Sar1bmut/mut embryos revealed no macroscopic abnormalities at E18.5. The gene discussed is MTTP; the disease is hypobetalipoproteinemia.