All eukaryotic cells express a 76-amino-acid protein known as ubiquitin. This protein has the unique ability to be covalently attached onto other proteins in an enzymatic process known as ubiquitination (alternatively ubiquitylation). Ubiquitination is involved in the posttranslational regulation of a wide array of cellular processes, and different types of ubiquitin modifications exist that in turn can promote distinct protein fates. The most well understood role for ubiquitination is to target modified proteins for destruction by the large proteolytic enzyme complex known as the 26S proteasome. Ubiquitination is also involved in altering protein localization, promoting protein-protein interactions, regulating gene expression, and changing the activity of cellular enzymes.