Classical pharmacology traditionally has been the basis for the discovery of new drugs.  Compounds are screened in cellular or animal models of disease to identify compounds that cause a desirable change in phenotype.  Only after the compounds have been discovered, an effort is made to determine the biological target of the compounds through target validation experiments often involving chemoproteomics. More recently it has become popular to develop a hypothesis that a certain biological target is disease modifying and screen for compounds that modulate the activity of this purified target.  Afterwards, these compounds are tested in animals to see if they have the desired effect.  This approach is known as "reverse pharmacology" or "target based drug discovery" (TDD). However, recent statistical analysis reveals that a disproportionate number of first-in-class drugs with novel mechanisms of action come from phenotypic screening, which has led to a resurgence of interest in this method.