UBE2E3 also known as UBCH9 or ubiquitin-conjugating enzyme E2E 3 is a member of the E2 ubiquitin-conjugating enzyme family. Ubiquitination, the covalent attachment of the protein ubiquitin (Ub) to other cellular proteins is an important cellular mechanism for targeting abnormal or short-lived proteins for degradation that involves at least three classes of enzymes: ubiquitin-activating enzymes, or E1s, ubiquitin-conjugating enzymes, or E2s, and ubiquitin-protein ligases, or E3s(1). UBE2E3 is highly expressed in skeletal muscle (2). UBE2E3 is essential for the proliferation of RPE-1 cells and is downregulated during RPE layer maturation in the developing mouse eye. These findings indicate that UBE2E3 is a major enzyme in modulating the balance between RPE cell proliferation and differentiation.