Ubiquitination is the covalent attachment of one or more ubiquitin molecules to a target protein. In endogenous protein breakdown, a chain of ubiquitin tags a protein for recognition and degradation by the proteasome. Targeting signals such as certain N-terminal residues or PEST sequences (often exposed or activated by phosphorylation) direct the ubiquitination machinery to the correct substrate, ensuring the right protein is degraded at the right time.