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:29868776, PubMed:27653677). 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:29868776, PubMed:27653677). Ufmylation is involved in reticulophagy (also called ER-phagy) induced in response to endoplasmic reticulum stress (PubMed:32160526). Ufmylation of TRIP4 regulates nuclear receptors-mediated transcription (PubMed:25219498).