Covalent modifications of proteins, such as phosphorylation, acetylation and ubiquitylation, play an important role in most cellular processes because they can cause rapid changes in the activities of pre-existing proteins. This type of mechanism for regulating protein function is especially crucial in signal transduction pathways and in cell cycle. The Ubiquitin System is one of the major protein-modification systems required for the highly selective turnover of specific proteins in eukaryotic cells. These ubiquitin-like proteins modulate protein function in the cell through reversible post-translational modification, which has been linked to several cellular regulatory pathways including cell-cycle progression, apoptosis, differentiation, intracellular targeting and responses to stress. One such ubiquitin-like protein is SUMO (Small Ubiquitin-Related Modifier) variously called as PIC1 (Phosphoinositidase-C1); UBL1 (Ubiquitin-like [...]