Inosine exists in mRNA at tissue-specific levels and is most abundant in brain mRNA.

The general view that mRNA does not contain inosine has been challenged by the discovery of adenosine deaminases that act on RNA (ADARs). Although inosine monophosphate (IMP) cannot be detected in crude preparations of nucleotides derived from poly(A)+ RNA, here we show it is readily detectable and quantifiable once it is purified away from the Watson-Crick nucleotides. We report that IMP is present in mRNA at tissue-specific levels that correlate with the levels of ADAR mRNA expression. The amount of IMP present in poly(A)+ RNA isolated from various mammalian tissues suggests adenosine deamination may play an important role in regulating gene expression, particularly in brain, where we estimate one IMP is present for every 17 000 ribonucleotides.