Post-transcriptional editing of apolipoprotein B (apoB) mRNA is regulated byAPOBEC1 (also designated human (or rat) small intestinal apolipoprotein B mRNA editing protein, HEPR or REPR) in hepatic cells to achieve a steady state proportion of edited and unedited RNA molecules. Two forms of apoB are known to circulate in the plasma of mammals. ApoB-100 is a protein primarily synthesized in the liver as a structural component of very low density lipoprotein particles. A truncated form of apoB-100, apoB-48, is synthesized in the small intestine and contains the amino-terminal 2,152 amino acids of the larger protein. This organ-specific partitioning of apoB production is the result of RNA editing of a common apoB gene.