Ubiquitin-conjugating Enzyme E2K (UBE2K), also known as UbcH1, E2-25K, HIP2 and LIG, is a 200 amino acid (aa) protein with a predicted molecular weight of 25 kDa. Human and mouse UbcH1 share 100% aa sequence identity. Similar to other ubiquitin-conjugating (E2) enzymes, UbcH1 contains a conserved ubiquitin-conjugating domain but is unique in that it also has a C-terminal, non-covalent ubiquitin binding surface termed the ubiquitin-associated domain between aa residues 160 and 200. UbcH1 mediates the elongation of Lys48-linked poly-ubiquitin chains. UbcH1 catalytic activity can be modulated by the post-translational addition of SUMO at Lys14. Substrates include the Huntingtin protein and the tumor suppressor RB1. UbcH1 is widely expressed, with highest levels found in the brain cortex and striatum, and dysregulated UbcH1 is implicated in polyglutamine diseases and Alzheimer's disease.