A site-specific cytidine deamination (cytidine to uridine) in nuclear apolipoprotein B messenger RNA creates a translational stop codon that produces apolipoprotein B48. This process is mediated by an enzyme composed of distinct subunits, including apolipoprotein B messenger RNA editing enzyme catalytic polypeptide-1 and additional complementation factors. The apolipoprotein B messenger RNA editing enzyme catalytic polypeptide-1 is expressed ubiquitously in the rat, but is largely confined to the small intestine in humans and rabbits. By contrast, complementation activity is present in tissues that neither express nor edit apolipoprotein B messenger RNA.