E3 ligases are a type of enzyme that play a crucial role in the process of ubiquitination, which is essential for regulating protein degradation in cells. They are responsible for transferring ubiquitin, a small protein, to target proteins, marking them for destruction by the proteasome. This process helps maintain cellular homeostasis by removing damaged or unneeded proteins. There are three main classes of E3 ligases: RING, HECT, and RBR. Each class has a unique mechanism for facilitating the transfer of ubiquitin. By controlling protein levels, E3 ligases are vital for various cellular processes, including cell cycle regulation, DNA repair, and responses to stress.