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.
Rats are used to make rat monoclonal anti mouse antibodies. There are less rat- than mouse clones however. Rats genes from rodents of the genus Rattus norvegicus are often studied in vivo as a model of human genes in Sprague-Dawley or Wistar rats.