The N-end rule relates the in vivo half-life of a protein to the composition of its N-terminal residues. The N-end rule pathway is part of the ubiquitin system, which involves a three-step mechanism. Proteins targeted for degradation are bound on their N-terminal residue by Ubr1 (also designated E3 α and N-recognin), which catalyzes the covalent attachment of ubiquitin to the protein substrate. Two zinc finger domains and the RING-H2 finger domain of Ubr1 are essential for substrate recognition. Ubr1 is located on mouse chromosome 2 and on human chromosome 15 in the syntenic region. Ubr1 is a 220 kDa protein that is ubiquitously expressed in adult mouse, with the highest expression detected in skeletal muscle and heart. In mouse embryo, Ubr1 is primarily expressed in the branchial arches and in the tail and limb buds.Synonyms: BUB1 beta, BUBR1, MAD3/BUB1-related protein kinase, MAD3L, Mitotic checkpoint kinase MAD3L, Mitotic checkpoint serine/threonine-protein kinase BUB1 beta, Protein SSK1