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.

RELION provides a GUI, several command-line tools in parallel (MPI) and serial versions, optionally with CUDA/GPU support.

relion-gui provides the graphical user interface without CUDA/GPU support.