The small ubiquitin-like modifier (SUMO) is involved in many cellular processes and is required for normal growth and development in all eukaryotes. Whereas lower eukaryotes have a single version of SUMO, higher eukaryotes have three versions: SUMO-1, -2 and -3. Similarly to most other ubiquitin-like proteins, the primary translation products of the SUMO genes need to be proteolytically processed to expose the C-terminal glycine that will be linked to lysine side chains in substrates. Processing of SUMO precursors is mediated by SUMO-specific proteases that also remove SUMO from modified proteins and depolymerise poly-SUMO chains.