Urm1 is a ubiquitin-like protein that is conjugated to substrates through a cascade of E1 activating enzymes, E2 conjugating enzymes, and E3 ligases. Conjugation of Urm1 to target proteins (urmylation) regulates their activity, stability, subcellular localization, or macromolecular interactions. The urmylation process can be significantly enhanced by oxidative stress. Urm1 also acts as a sulfur carrier in the thiolation of eukaryotic tRNA. (Adapted from PMID:28234446 and PMID:21904977.)