Classical Isosteres are molecules or ions with similar shape and often electronic properties. Many definitions are available.[1] but the term is usually employed in the context of bioactivity and drug development. Such biologically-active compounds containing an isostere is called a bioisostere. This is frequently used in drug design:[2] the bioisostere will still be recognized and accepted by the body, but its functions there will be altered as compared to the parent molecule.