Ubiquitin-like protein which can be covalently attached to target lysines as a monomer. Does not seem to be involved in protein degradation and may function as an antagonist of ubiquitin in the degradation process (By similarity). Interacts with SAE2, SCE1 and SIZ1. Covalently attached to a number of proteins (By similarity). Belongs to the ubiquitin family. SUMO subfamily.