Types of Targets Ion Channels Ion channels represent a class of membrane spanning protein pores that mediate the flux of ions in a variety of cell types. To date, more than 400 ion channels have been cloned and characterized, and some of these channels have emerged as attractive drug targets. Several existing medications elicit their therapeutic effect through the modulation of ion channels, underscoring the importance of ion channels as a target class for modern drug discovery. GPCRs The GPCRs are the largest protein family known, members of which are involved in all types of stimulus-response pathways from intercellular communication to physiological senses. Their diversity of function is matched by the wide range of ligands recognized by members of the family, from photons (rhodopsin, the archetypal GPCR) to small molecules (in the case of the histamine receptors) to proteins (for example, chemokine receptors). This pervasive involvement in normal biological processes has the consequence of involving GPCRs in many pathological conditions. GPCRs are the target of 40 to 50% of modern medicinal drugs. Types of Assays Absorbance Absorbance is the amount of light absorbed by a solution. To measure absorbance, one uses a spectrophotometer to determine the amount of light passing through the sample relative to the total amount of light available. Absorbance measurements are commonly used to determine the concentration of proteins and DNA. Colorimetric assays, those that generate a colored product like the reaction of beta galactosidase with X-Gal (5-bromo-4-chloro-3-indolyl-b -galactopyranoside), are measured using absorbance. Fluorescence Fluorescence occurs when a substance absorbs light energy of characteristic wavelength (excitation), undergoes an electronic state change and emits light of a longer wavelength (emission). Most fluorescent material used in HTS have well-characterized excitation and emission spectra and include popular dyes like fluorescein and rhodamine. Time Resolved Fluorescence Normally fluorescence readings are taken with the light source on, however excitation energy or background fluorescence can contaminate the emission signal. To eliminate the background excitation, time resolved fluorescence is preformed by flashing the light source, turning if off and measuring the delayed emission. Long lifetime rare-earth lanthanide dyes are typically used for these experiments. Fluorescence Polarization This technique measures molecular orientation and mobility using polarized light and a fluorescent tracer. Molecules spin randomly in solution and the larger the molecule the slower it spins. When a molecule interacts with another molecule, its rotation in solution slows and the light emitted becomes direction dependent (anisotropy). Fluorescence polarization techniques measure the polarization and anisotropy of emitted light. This assay techniques can be used to study protein-protein interactions for example. Luminescence Luminescence is the emission of light by processes that derive energy from nonthermal changes. Luminescence assays include the use of reporters like green fluorescent protein (GFP) and luciferase.