Small ubiquitin-related modifier (SUMO) family contains a series of ubiquitin-like proteins, such as SUMO1, SUMO2, SUMO3, SUMO4, SUMO5 and SUMO8. The SUMO protein and ubiquitin have similar 3-D structure. Sumolyzation and ubiquitination exhibit similar biological processes for post-translational modification. However, unlike ubiquitination, which targets proteins for degradation, sumolyzation participates in a number of cellular processes such as nuclear transport, transcriptional regulation, apoptosis and protein stability.