There are various similarities between cAMP and cGMP. One similarity is that they are both types of cyclic nucleotides. They both act as second messengers in both ion-channel and hormone signaling in eukaryotes. Another similarity is that they both act as allosteric effector compounds of DNA-binding proteins in prokaryotes. cAMP and cGMP both have a wide range of protein-ligand interactions, and are able to activate protein kinases.