The ADAR (Adenosine Deaminase Acting on RNA) enzyme family is responsible for the most common form of RNA editing in metazoans. It catalyzes the deamination of Adenosine into Inosine and Inosine is read as Guanosine during translation thus resulting in nonsynonymous recoding of the protein sequence. However, RNA editing rarely takes place in the coding region. The vast majority of the editing sites lie in untranslated regions and only 3% of mRNAs are edited. This distribution suggests the presence of an active mechanism that exclude editing sites from coding regions.
Contrastingly, the coleoid cephalopods exhibit unusually high levels of RNA editing. The common market squid edits two-thirds of its neural messages and the octopus and cuttlefish are said to recode a similar amount.
What is even more astonishing about this process is that editing of coding RNA at such a massive scale slows down the rate of DNA-level evolution. For editing a specific adenosine, ADAR requires the support of surrounding RNA structures. Even a single substitution in these regions would put a stop to the editing. Therefore thousands of stretches of sequences that span a significant portion of the genomic coding sequence are highly conserved. Consequently, the accumulation of mutations is reduced, which has a detrimental effect on genetic variation which in turn slows down conventional DNA-level evolution.
For cephalopods to recode their mRNAs at a massive scale by forfeiting DNA-level evolution suggests that RNA editing provides them with a colossal advantage. What that advantage is a question that is still unanswered.
References: Liscovitch-Brauer, N., et al.,(2017). Trade-off between Transcriptome Plasticity and Genome Evolution in Cephalopods. Cell, 169(2), 191-202.e11. https://doi.org/10.1016/j.cell.2017.03.025