The mechanism for apo-B mRNA editing is deamination.

Apolipoprotein (apo-) B mRNA editing at nucleotide 6666 converts cytidine to uridine, transforming the codon for glutamine-2153 to a termination codon. To investigate this editing mechanism, [a-32P] and [5-3H] CTP were incorporated into synthetic apo-B RNA. After the substrate had been edited extensively in vitro by a partially purified editing extract, the edited base was isolated and analyzed for radioactivity. The uridine-6666 resulting from the editing reaction had the same ratio of 3H to 32P as did the cytidine-6666, demonstrating that deamination rather than base exchange or nucleotide replacement is the mechanism for apo-B mRNA editing.