50% of drugs) enzymes (especially protein kinases, proteases, esterases, and phosphatases) ion channels ligand-gated ion channels voltage-gated ion channels nuclear hormone receptors structural proteins such as tubulin membrane transport proteins nucleic acids Drug target identification Identifying the biological origin of a disease, and the potential targets for intervention, is the first step in the discovery of a medicine using the reverse pharmacology approach. Potential drug targets are not necessarily disease causing but must by definition be disease modifying. An alternative means of identifying new drug targets is forward pharmacology based on phenotypic screening to identify "orphan" ligands whose targets are subsequently identified through target deconvolution. Databases Databases containing biological targets information: Therapeutic Targets Database (TTD) DrugBank Binding DB Conservation ecology These biological targets are conserved across species, making pharmaceutical pollution of the environment a danger to species who possess the same targets. For example, the synthetic estrogen in human contraceptives, 17-R-ethinylestradiol, has been shown to increase the feminization of fish downstream from sewage treatment plants, thereby unbalancing reproduction and creating an additional selective pressure on fish survival. Pharmaceuticals are usually found at ng/L to low-μg/L concentrations in the aquatic environment. Adverse effects may occur in non-target species as a consequence of specific drug target interactions. Therefore, evolutionarily well-conserved drug targets are likely to be associated with an increased risk for non-targeted pharmacological effects. See also Drug discovery Environmental impact of pharmaceuticals and personal care products References Pharmacology Biology terminology