SUMOylation is a post-translational modification that plays a role in various cellular processes, such as nuclear-cytosolic transport, transcriptional regulation, apoptosis, protein stability, response to stress, and progression through the cell cycle. Small Ubiquitin-like Modifier (SUMO) proteins are attached to and detached from other proteins in cells to alter their function. The detection of in vivo protein SUMOylation (SUMO conjugation) would provide useful information for understanding the SUMO modification that emerges as an important control mechanism for regulating the activity of many nuclear proteins.