Chirality is an important concept for stereochemistry and biochemistry. Tacticity describes the relative stereochemistry of chiral centers in neighboring structural units within a macromolecule. Wavy single bonds represent unknown or unspecified stereochemistry or a mixture of isomers. An important branch of stereochemistry is the study of chiral molecules. The isomer distribution depends on the stereochemistry of the epoxide. Alkane conformers are one of the subjects of alkane stereochemistry. Initially, the approach depends upon an enantioselective Sharpless epoxidation, which sets the stereochemistry at C20. The absolute stereochemistry at these carbons is responsible for vincristine's anticancer activity. An equation that addresses stereochemistry in aliphatic systems has also been developed. Hyaluronic acid is energetically stable, in part because of the stereochemistry of its component disaccharides. Prelog priority rules are part of a system for describing a molecule's stereochemistry. The descriptors only describe relative stereochemistry rather than absolute stereochemistry. Such reaction is highly selective in stereochemistry. Note also the example of the stereochemistry of ketonization of enols and enolates. This creates the demand for 'stereoselective synthesis', producing a compound with a defined stereochemistry.