Currently, receptor modulators are categorized in the Agonist, Partial Agonist, Selective Tissue Modulators, Antagonist, and Inverse Agonist categories in terms of the effect they cause. They are further divided into Orthosteric or Allosteric Modulators according to how they effect said result. Typically, a chemical acts in an agonist fashion whenever it instigates or else facilitates a particular reaction by binding to a particular receptor. In contract, a chemical acts as an antagonist whenever binding to a particular receptor blocks or inhibits a particular response. Between these endpoints exists a gradient defined by a number of variables. One example is Selective Tissue Modulators, which mean a given ligand can behave differently according to the tissue type it is in. As for orthosteric and allosteric modulation, this describes the manner in which the ligand binds to the receptor in question: if it binds directly to the prescribed binding site of a receptor, the ligand is orthosteric in this instance; if the ligand alters the receptor by interacting with it at any place other than a binding site, allosteric interaction occurred. Note that a drug's categorization does not dictate how another drug of the same family could be categorized or whether the same drug may also function in another category. An example is found in medications used to treat opioid addiction, with methadone, buprenorphine, naloxone, and naltrexone all in separate categories or in more than one simultaneously. In addition, depending on the cell type, the specific effect, whether agonist, antagonist, inverse agonist, etc., could have a unique specific effect. An example is seen in insulin, under "Receptor Agonists," as it interacts with multiple different cell types as an agonist, but incites multiple and different responses in both.