Protein ubiquitination is a cellular process where small proteins called ubiquitin molecules are attached to a target protein. This modification typically signals for the protein to be degraded by the proteasome, a complex that breaks down unwanted or damaged proteins. Ubiquitination plays a crucial role in regulating various cellular functions, including the cell cycle, DNA repair, and responses to stress. The process of ubiquitination involves a series of enzymatic reactions, primarily carried out by three types of enzymes: E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme), and E3 (ubiquitin ligase). The specificity of the process is largely determined by the E3 enzyme, which recognizes the target protein and facilitates the transfer of ubiquitin. This regulation is vital for maintaining cellular homeostasis and function.