Hopanoids are a diverse subclass of triterpenoids with the same hydrocarbon skeleton as the compound hopane. This group of pentacyclic molecules therefore refers to simple hopenes, hopanols and hopanes, but also to extensively functionalized derivatives such as bacteriohopanepolyols (BHPs) and hopanoids covalently attached to lipid A. The first known hopanoid, hydroxyhopanone, was isolated by two chemists at The National Gallery, London working on the chemistry of dammar gum, a natural resin used as a varnish for paintings. While hopanoids are often assumed to be made only in bacteria, their name actually comes from the abundance of hopanoid compounds in the resin of plants from the genus Hopea. In turn, this genus is named after John Hope, the first Regius Keeper of the Royal Botanic Garden, Edinburgh. Since their initial discovery in an angiosperm, hopanoids have been found in plasma membranes of bacteria, lichens, bryophytes, ferns, tropical trees and fungi. Hopanoids have stable polycyclic structures that are well-preserved in petroleum reservoirs, rocks and sediment, allowing the diagenetic products of these molecules to be interpreted as biomarkers for the presence of specific microbes and potentially for chemical or physical conditions at the time of deposition. Hopanoids have not been detected in archaea. Biological function About 10% of sequenced bacterial genomes have a putative shc gene encoding a squalene-hopene cyclase and can presumably make hopanoids, which have been shown to play diverse roles in the plasma membrane and may allow some organisms to adapt in extreme environments. Since hopanoids modify plasma membrane properties in bacteria, they are frequently compared to sterols (e.g., cholesterol), which modulate membrane fluidity and serve other functions in eukaryotes. Although hopanoids do not rescue sterol deficiency, they are thought to increase membrane rigidity and decrease permeability. Also, gammaproteobacteria and eukaryotic organisms such as lichens and bryophytes have been shown to produce both sterols and hopanoids, suggesting these lipids may have other distinct functions. Notably, the way hopanoids pack into the plasma membrane can change depending on what functional groups are attached. The hopanoid bacteriohopanetetrol assumes a transverse orientation in lipid bilayers, but diploptene localizes between the inner and outer leaflet, presumably thickening the membrane to decrease permeability. The hopanoid diplopterol orders membranes by interacting with lipid A, a common membrane lipid in bacteria, in ways similar to how cholesterol and sphingolipids interact in eukaryotic plasma membranes. Diplopterol and cholesterol were demonstrated to promote condensation and inhibit gel-phase formation in