Ubiquitination is a cellular process where a small protein called ubiquitin is attached to a target protein. This modification can signal the target protein for various outcomes, such as degradation, altering its activity, or changing its location within the cell. Ubiquitination plays a crucial role in regulating many cellular functions, including the cell cycle and response to stress. The process involves a series of enzymatic reactions carried out by three main types of enzymes: E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme), and E3 (ubiquitin ligase). The E3 enzyme is particularly important as it determines the specificity of the ubiquitination process, ensuring that the right proteins are modified in response to cellular signals.