UBE2I and acute promyelocytic leukemia: Mechanistically in APL cells ATO directly binds to the cysteine residues in the zinc-finger domain of the RBCC domain of PML/RARα and PML and brings them to small ubiquitin-like modifier (SUMO)-conjugating enzyme UBC9 for SUMOylation followed by degradation, leading to APL cell differentiation5.