Ubiquitin-like protein which can be covalently attached to target lysines either as a monomer or as a lysine-linked polymer. Does not seem to be involved in protein degradation and may function as an antagonist of ubiquitin in the degradation process. Plays a role in a number of cellular processes such as nuclear transport; DNA replication and repair; mitosis and signal transduction. Covalent attachment to its substrates requires prior activation by the E1 complex SAE1-SAE2 and linkage to the E2 enzyme UBE2I; and can be promoted by an E3 ligase such as PIAS1-4; RANBP2 or CBX4 (PubMed:11451954; PubMed:18538659; PubMed:21965678). Plays a role in the regulation of sumoylation status of SETX (PubMed:24105744). {ECO:0000269|PubMed:11451954; ECO:0000269|PubMed:18538659; ECO:0000269|PubMed:21965678}.