Ubiquitin-like modifier which can be covalently attached via an isopeptide bond to lysine residues of substrate proteins as a monomer or a lysine-linked polymer (PubMed:15071506, PubMed:20018847, PubMed:27653677, PubMed:29868776, PubMed:30626644, PubMed:38377992, PubMed:38383785). The so-called ufmylation, requires the UFM1- activating E1 enzyme UBA5, the UFM1-conjugating E2 enzyme UFC1, and the UFM1-ligase E3 enzyme UFL1 (PubMed:15071506, PubMed:20018847, PubMed:27653677, PubMed:29868776). Ufmylation is involved in various processes, such as ribosome recycling, response to DNA damage, transcription or reticulophagy (also called ER-phagy) induced in response to endoplasmic reticulum stress (PubMed:25219498, PubMed:32160526, PubMed:38383785). {ECO:0000269|PubMed:15071506, ECO:0000269|PubMed:20018847, ECO:0000269|PubMed:25219498, ECO:0000269|PubMed:27653677, ECO:0000269|PubMed:29868776, ECO:0000269|PubMed:30626644, ECO:0000269|PubMed:32160526, ECO:0000269|PubMed:38377992, ECO:0000269|PubMed:38383785}. This is the function of UFM1 (ubiquitin fold modifier 1, Ensembl gene identifier ENSG00000120686).