The orderly patterns of metabolic and developmental reactions giving rise to the unique characteristics of the individual and of its species. It is one of the most widespread and characteristic properties of living organisms. Biological specificity is most pronounced and best understood at the cellular and molecular levels of organization, where the shapes of individual molecules allow them to selectively recognize and bind to one another. The main principle which guides this recognition is termed complementarity. Just as a hand fits perfectly into a glove, molecules which are complementary have mirror-image shapes that allow them to selectively bind to each other.