All four receptor types have been cloned and belong to the large superfamily of receptors having seven membrane spanning regions and coupled with g pritiens. The seucture of h1 and h2 receptors differ significantly and appear to be more closely related to muscarinic and 5-ht1 receptors, respectively, than to each other. The H4 receptor has about 40% homology with the h3 receptor but does not seem to be closely related to any other histamine receptor.