Apolipoprotein B (apoB) gene expression is regulated by transcriptional, posttranscriptional, and translational mechanisms in both small intestinal enterocytes and hepatocytes of all mammals. This review focuses on the posttranscriptional regulation of apoB gene expression - specifically, an enzymatic modification to the nuclear transcript referred to as C to U RNA editing. C to U RNA editing, the process responsible for the production of apoB-48, is an exquisitely precise process that targets a single nucleotide in over 14,000. RNA editing requires regulated interactions between core enzyme components whose identity is known and regulatory factors, most of which have yet to be fully characterized. This review summarizes information relevant to those transacting factors that are essential for editing activity (core factors) as well as those interacting partners (regulatory factors) whose physiological role in RNA editing is yet to be fully defined.