Ubiquitin (Ub) and Ub-like proteins are typified by a beta-grasp fold. Ub-like proteins (Ubl) such as Nedd8 and SUMO are also covalently linked to target polypeptides, just as Ub itself. This fold is also found in Ub-related domains, such as the UBX domain and Ras-binding domain (RBD).