RELION (for REgularised LIkelihood OptimisatioN) is a stand-alone computer program for Maximum A Posteriori refinement of (multiple) 3D reconstructions or 2D class averages in cryo-electron microscopy. It is developed by the research group of Sjors Scheres at the MRC Laboratory of Molecular Biology. The underlying theory of MAP refinement is given in Scheres (2011), submitted. If RELION is useful in your work, please cite this paper! Thank you.