Many biomolecules-nucleic acids, proteins, polysaccharides, lipids, and many drug molecules-are chiral enantiomers, or stereoisomers that are mirror images of each other (Pasteur 1901; Schaus 2000; Caldwell 2001; Wainer 2001). A 50:50 mixture of chiral enantiomers is called a “racemate.” The enantiomers have identical physical and chemical properties. The human body contains, however, numerous chiral sites that show stereospeciˆc interactions with only one enantiomer and may metabolize each enantiomer by separate pathways to produce different pharmacological activities. One enantiomer may be safe and therapeutically effective, while another may be toxic because of slow metabolism and accumulation in internal organs.