Photoactivated guanylyl cyclases have been discovered in the aquatic fungi Blastocladiella emersonii and Catenaria anguillulae. Unlike PACs, these light-activated cyclases use retinal as their light sensor and are therefore rhodopsin guanylyl cyclases (RhGC). When expressed in Xenopus oocytes or mammalian neurons, RhGCs generate cGMP in response to green light. Therefore, they are considered useful optogenetic tools to investigate cGMP signaling.