Monoubiquitination of cell surface receptors is a sorting signal that leads to receptor trafficking from endosomes to lysosomes. Ubiquitinated protein sorting is carried out by class E vacuolar protein sorting (Vps) proteins. Some of these proteins are regulated by monoubiquitination. The Hrs-STAM complex, which is essential for the initial step of the sorting pathway, binds the deubiquitinating enzymes USP8 (UBPY) and STAMBP (AMSH). These bind STAM2 at the same site and may compete for binding (Kato et al. 2000). STAM2 stability is dramatically reduced in USP8 knockdown cells suggesting that its degradation is reduced by the DUB action of USP8 (Row et al. 2006).