HR6B/UBE2B: ProductsHomolog of Rad6 B (HR6B), also known as Ubiquitin-conjugating Enzyme E2B (UBE2B), is a ubiquitously expressed member of the Ubiquitin-conjugating (E2) enzyme family. It has a predicted molecular weight of 17 kDa. The human enzyme shares 100% amino acid sequence identity with the mouse and rat orthologs. HR6B/UBE2B localizes to the nucleus and cytoplasm and is highly expressed in the brain, heart, and testis. HR6B/UBE2B has an E2 catalytic core domain with an active site cysteine residue that is required for the formation of a thioester bond with Ubiquitin. PCNA is mono-ubiquitinated by HR6B/UBE2B via the RAD18 Ubiquitin ligase (E3). HR6B/UBE2B also functions with the UBR2 E3 to ubiquitinate Histone H2A. HR6B/UBE2B plays important roles during spermatogenesis and is required for fertility in male mice. Pathologically, HR6B/UBE2B is overexpressed in mouse and human breast cancer cell lines and tumors and has been reported to provide resistance to chemotherapy drugs.