The RIO are named after 'right open reading frame', as the founding member was one of two adjacent genes that was found to be transcribed divergently from the same intergenic region. The RIO are a small group of essential aPKs found in all eukaryotic species analysed to date.

The three-dimensional structures of Archaeglobus fulgidus Rio1 and Rio2 have shown that Rio1 and Rio2 share the overall fold of ePKs, although some of the kinase structural domains, particularly those involved in peptide substrate binding, are not present. Rio1 and Rio2 also differ from other kinases as to the mode of binding of nucleotides. In S. cerevisiae, the enzymatically active Rio1 and Rio2 are essential and required for proper cell cycle progression and chromosome maintenance, but the identity of their biological substrates remains elusive, though some RIO kinases have been shown to autophosphorylate.