The SUMO (small ubiquitin-like modifier) 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. The human genome contains three SUMO-1/2/3 functional genes, as well as eight SUMO-1 pseudogenes and 23 SUMO-2 pseudogenes, but no SUMO-3 pseudogenes. The protein-coding sequence of the SUMO-1 gene is interrupted by four introns, while those of SUMO-2 and SUMO-3 genes are interrupted by three introns. Human SUMO-1 protein exhibits 44% sequence identity with SUMO-2 and SUMO-3 proteins, while SUMO-2 and SUMO-3 proteins share 86% sequence identity. Phylogenetic analyses indicate that the SUMO-3 gene was derived from the SUMO-2 gene. SUMO-1 mRNA appears to be most abundant in human epithelial HeLa, kidney 293 and neuronal NT2 cells, while the SUMO-3 mRNA seems to be much less abundant than SUMO-2 mRNA, especially in HeLa and 293 cells. Many cellular proteins of high molecular weights were covalently modified by SUMO-1/2/3 proteins. However, some free form of SUMO-2/3 proteins was also detected. Most SUMO-1/2/3 proteins were shown to be localized on nuclear membrane, nuclear bodies and cytoplasm, respectively.