The viral envelope consists of a lipid bilayer, in which the membrane (M), envelope (E) and spike (S) structural proteins are anchored.[45] The ratio of E:S:M in the lipid bilayer is approximately 1:20:300.[46] On average a coronavirus particle has 74 surface spikes.[47] A subset of coronaviruses (specifically the members of betacoronavirus subgroup A) also have a shorter spike-like surface protein called hemagglutinin esterase (HE).[4]