Fluorescent markers, sometimes known as fluorophores, is any molecule with the ability to absorb light and emit it at some other well-defined wavelength. Fluorophores come in a variety of types ranging from simple organic compounds to large proteins like Green Fluorescent Protein. Fluorescent markers give the ability to investigate proteins in their biological environment. When the light of a certain wavelength is directed at the molecule's chromophore, a photon is absorbed and excites an electron to a higher energy state. The electron then relaxes back to its ground state. The energy that is released is determined by the formula E = hν, where h is Planck's constant, and ν is the frequency of the photon. This energy can then be related back to the wavelength of the emitted photon, which corresponds to a specific color on the visible spectrum. The difference between the absorption and emission wavelength is called the Stokes shift. Large Stokes shifts are generally desirable because the emitted light from the fluorescent tag can be filtered out more easily from the excitation light. Fluorescence educing molecules such as FITC (fluorescein isothiocyanate) are used to stain cells which give the ability to examine these cells under a fluorescence microscope. Multiple fluorescent markers can be used to stain different parts of the cell. Fluorescence microscopy can aid in determining the location of specific proteins within a cell. One can even track the movement of these proteins and derive possible functions for these proteins of interest.