C-to-U editing of the mRNA for ApoB protein. (a) Cytidine deaminase catalyzes the deamination of cytidine to uridine. (b) The ApoB gene is expressed in the liver and intestine. In the intestine, the mRNA goes through an additional processing step (after splicing) in which a C residue is changed to a U residue by cytidine deaminase, resulting in a stop codon and producing a smaller protein (ApoB48) that aids in the intestinal absorption of lipids.