Ubiquitin is a small protein found in all eukaryotes (most eukaryotic cells). Its main function is to label proteins that need to be broken down to be degraded by the 26S proteasome. Ubiquitin can also be labeled transmembrane proteins that are involved in membrane vesicle transport of proteins. The atypical ubiquitin chain plays an important role in cell signaling, endocytosis and DNA damage repair, and regulates the NF-κB pathway.