Figure 38.12 RNA editing. Enzyme-catalyzed deamination of a specific cytidine residue in the mRNA for apolipoprotein B−100 changes a codon for glutamine (CAA) to a Stop codon (UAA). Apolipoprotein B−48, a truncated version of the protein lacking the LDL receptor-binding domain, is generated by this posttranscriptional change in the mRNA sequence.