Ubiquitin-protein ligase E3A (UBE3A) also known as E6AP ubiquitin-protein ligase (E6AP) is an enzyme that in humans is encoded by the UBE3A gene. Ubiquitin protein ligase E3A attaches a small marker protein called ubiquitin to proteins that should be degraded. Cellular structures called proteasomes recognize and digest proteins tagged with ubiquitin. Mutations within the UBE3A gene are responsible for some cases of Angelman syndrome and Prader-Willi syndrome. Most of these mutations result in an abnormally short, nonfunctional version of ubiquitin protein ligase E3A. UBE3A associates with the E6 protein of certain strains of HPV. This interaction promotes the polyubiquitination and subsequent degradation of the tumor suppressor gene p53, thereby enabling the immortalization of infected cells. Strains of HPV with this ability have a higher risk of causing HPV-associated cancers.