Substitutions can occur on the level of both the DNA and RNA. RNA editing-dependent amino acid substitutions can produce missense mRNA's of which occur through hydrolytic deaminase reactions. Two of the most prevalent deaminase reactions occur through the Apolipoprotein B mRNA editing enzyme (APOBEC) and the adenosine deaminase acting on RNA enzyme (ADAR) which are responsible for the conversion of cytidine to uridine (C-to-U), and the deamination of adenosine to inosine (A-to-I), respectively. Such selective substitutions of uridine for cytidine, and inosine for adenosine  in RNA editing can produce differential isoforms of missense mRNA transcripts, and confer transcriptome diversity and enhanced protein function in response to selective pressures.