 HxA3, HxB3, and their isomers are distinguished from most other eicosanoids (i.e.signaling molecules made by oxidation of 20-carbon fatty acids) in that they contain both epoxide and hydroxyl residues; they are structurally differentiated in particular from two other classes of arachidonic acid-derived eicosanoids, the leukotrienes and lipoxins, in that they lack conjugated double bonds.HxA4 and HxB4 are distinguished from HxA3 and HxB3 by possessing four rather than three double bonds.