The receptor guanylate cyclases are a small ePK group similar in sequence to the Tyrosine Kinase group found in metazoa but absent from fungi, plants and protists. The RGC group therefore represents a late innovation in the protein kinase superfamily. The photoreceptor membrane guanylate cyclase domain has Mg2+-dependent serine auto-phosphorylating kinase activity. Its kinase activity is unaffected by Ca2+, cyclic nucleotides and phorbols, but is inhibited by high concentrations of staurosporine. These properties are distinct from other serine/threonine kinases.