Adenosine deaminases acting on RNA (ADARs) can deaminate adenosine to form inosine. In long double-stranded RNA, this process is non-specific; it occurs site-specifically in RNA transcripts. The former is important in defence against viruses, whereas the latter may affect splicing or untranslated regions. They are primarily nuclear proteins, but a longer isoform of ADAR1 is found predominantly in the cytoplasm. ADARs are derived from the Tad1-like tRNA deaminases that are present across eukaryotes. These in turn belong to the nucleotide/nucleic acid deaminase superfamily and are characterized by a distinct insert between the two conserved cysteines that are involved in binding zinc [2].

1: Adenosine deaminases acting on RNA (ADARs): RNA-editing enzymes. Keegan LP, Leroy A, Sproul D, O'Connell MA; Genome Biol 2004;5:209. PMID:14759252

2: Evolution of the deaminase fold and multiple origins of eukaryotic editing and mutagenic nucleic acid deaminases from bacterial toxin systems. Iyer LM, Zhang D, Rogozin IB, Aravind L; Nucleic Acids Res. 2011; [Epub ahead of print] PMID:21890906