Ubiquitin-like protein that can be covalently attached to proteins as a monomer or a lysine-linked polymer. Covalent attachment via an isopeptide bond to its substrates requires prior activation by the E1 complex SAE1-SAE2 and linkage to the E2 enzyme UBE2I; and can be promoted by E3 ligases such as PIAS1-4; RANBP2 or CBX4. This post-translational modification on lysine residues of proteins plays a crucial role in a number of cellular processes such as nuclear transport; DNA replication and repair; mitosis and signal transduction. Involved for instance in targeting RANGAP1 to the nuclear pore complex protein RANBP2. Polymeric SUMO1 chains are also susceptible to polyubiquitination which functions as a signal for proteasomal degradation of modified proteins. May also regulate a network of genes involved in palate development. {ECO:0000269|PubMed:18408734; ECO:0000269|PubMed:18538659; ECO:0000269|PubMed:21965678; ECO:0000269|PubMed:9019411; ECO:0000269|PubMed:9162015}.