Fluorescent proteins (FPs) are useful marker tools to visualize cellular structures. They are available in a range of the whole visible spectrum. FPs emitting in the red region of the spectrum (e.g. mRuby, RFP639) are of particular interest for life-cell imaging, because of less radiation damage by excitation light, separation of emission from cell autofluorescence background and little attenuation of emission by scattering and absorption. Some FPs have certain switching properties; they can be photoconverted from an ‘off’ into an ‘on’ state (e.g mEosFP) and are therefore suitable for super-resolution microscopy methods like PALM.