Diagram illustrating an assay for UBE2R2's di-ubiquitin chain formation activity in the presence of unanchored acceptor ubiquitin and comparison with either neddylated CUL2-RBX1 or neddylated CRL2FEM1Ccomplex. UBD, donor ubiquitin; UBA, acceptor ubiquitin; ELO B/C, ELONGIN B/C.