SUMO-1 is an 11kD protein which structurally related to ubiquitin, and is added to proteins as a form of post-translational modification known as sumoylation. Sumoylation can be cell cycle regulated, stimulated upon stress or induced by DNA damage, and binding elicits individual effects for each protein. Effects include changes to protein stability, nuclear-cytosolic transport, and transcriptional regulation (mostly transcriptional repression).