Ubiquitination is a cellular process where a small protein called ubiquitin is attached to a target protein. This tagging usually marks the target protein for degradation 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 several steps, including the activation of ubiquitin, its conjugation to the target protein, and the ligation process facilitated by ubiquitin ligases. This modification can also influence protein localization, activity, and interactions, making it essential for maintaining cellular homeostasis.