Ubiquitin can be covalently linked to many cellular proteins by the ubiquitination process, which targets proteins for degradation by the 26S proteasome. Three components are involved in the target protein-ubiquitin conjugation process. Ubiquitin is first activated by forming a thiolester complex with the ubiquitin-activating enzyme (UBE1 or E1). The activated ubiquitin is subsequently transferred to the ubiquitin-carrier protein E2, and then from E2 to ubiquitin ligase E3 for final delivery to the epsilon-amino group of the target protein lysine residue (1-3). Combinatorial interactions of different E2 and E3 proteins result in substrate specificity (4). UBE1 has two isofoms: UBE1a is a nuclear protein of 117kD while UBE1b is a nuclear and cytoplasmic protein of 110kD (5).