The gene encoding human apo B has been mapped to the short arm of chromosome 2 in the p23-p24 region. The apo B gene extends over 43 kb and is composed of 29 exons and 28 introns. Both apo B 100 and apo B 48 were encoded by the same gene. All intestinal cDNA clones contained a single C to T base substitution in the codon CAA encoding Gln2153 in apo B 100 cDNA, resulting in a translational stop. In human intestinal cells, apo B mRNA is recognized by a specific enzyme that modifies cytosine 6666 to a uracil, introducing a stop codon. Recently, a human apo B mRNA editing protein was cloned. The cDNA sequence predicts a translation product of 236-aa residues. The human apo B mRNA editing protein is a cytidine deaminase and exists as a homodimer. Familial hypobetalipoproteinemia can be caused by mutations in the apoB gene that interfere with the translation of a full-length apo B molecule. Frequently, a truncated apo B molecule can be detected in the plasma lipoproteins of familial hypobetalipoproteinemia. Abetalipoproteinemia is caused by defect of the gene encoding the large subunit of a microsomal triglyceride transfer protein.