Ubiquitin like proteins fall into two classes: the first class, ubiquitin-like modifiers (UBLs) function as modifiers in a manner analogous to that of ubiquitin. Examples of UBLs are SUMO, Rub1 (also called Nedd8), Apg12 and Hub1. Proteins of the second class include Parkin, RAD23 and DSK2, are designated ubiquitin domain proteins (UDPs). These proteins contain domains that are related to ubiquitin but are otherwise unrelated to each other. In contrast to UBLs, UDPs are not proteolytically processed or conjugated to other proteins. Rub1 and the corresponding human homolog NEDD8 are activated by the E1 ubiquitin activating enzyme UBA2, that forms isopeptide linkages between thio esters. NEDD8 shows 80% homology to ubiquitin. The best known targets of Rub1 modification are members of the cullin family. Cullins are subunits of an E3 ubiquitin ligase complex called the Skp1 /Cul1/Cdc53 F box (SCF). The SCF promotes transfer of ubiquitin from a ubiquitin conjugating enzyme (E2) to the target protein. Rub1 modification may regulate SCF function or localization.