Tissue, pathway, proteinase, peptidase, protease ,acrosin, lipoprotein, activator, caspase, trypsin, papain, esterase inhibitors are proteins or receptor ligands or receptor antagonists that bind to an enzyme receptor and decreases its activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. Not all receptor antagonist that bind to enzymes are inhibitors; enzyme activator ligands or agonists bind to enzymes and increase their enzymatic activity, while enzyme substrates bind and are converted to products in the normal catalytic cycle of the enzyme.Rhesus Monkeys proteins are often measured by ELISA on serum or plasma since consensus epitopes with the human ELISA are used for producing the antibodies of these ELISA test kits. Often cDNA of monkeys is used as alternative to human cDNA as a model for drug development. Monkeys are generally considered to be intelligent, particularly Old World monkeys.