UBE3D (ubiquitin protein ligase E3D) is an E3 ubiquitin ligase that catalyzes ubiquitin transfer from E2 enzymes to protein substrates, promoting their proteasomal degradation 1. Beyond its canonical ligase function, UBE3D acts as a cytoplasmic binding partner and inhibitor of CPSF3 endonuclease, blocking its active site through direct coordination of catalytic metal ions via a conserved cysteine residue 2. In this capacity, UBE3D functions as a cytoplasmic chaperone required for CPSF3's nuclear function in pre-mRNA 3' end processing 2. UBE3D also de-ubiquitinates CPSF3, maintaining its cellular levels 3. UBE3D is critical for embryonic development; knockout mice exhibit severe growth retardation, neural tube defects, and lethality (E10.5-E11.5), with disrupted expression of homeobox genes essential for forebrain and neural tube development 3. In the retina, UBE3D regulates age-related macular degeneration (AMD) pathogenesis through ubiquitin-proteasome system function; genetic variants associate with neovascular AMD in East Asian populations 45, and conditional RPE-specific knockout accelerates age-associated retinal degeneration involving oxidative stress and autophagy dysfunction 6. Additionally, UBE3D regulates 3βHSD1 homeostasis, influencing DHEA metabolism and prostate cancer aggressiveness 7.