RING finger proteins are essential mediators of ubiquitin ligase activity. Initially, the RING finger was thought to be involved only in protein dimerization, but recent studies have shown that RING finger proteins play critical roles in transferring ubiquitin (Ub) to both heterologous substrates and themselves. With hundreds of RING finger-containing proteins, the number of potential Ub protein ligases (E3s) has increased significantly. Ubiquitination begins with the formation of a thiol-ester linkage between the C terminus of Ub and the active site cysteine of the Ub activating enzyme (E1). Ub is then transferred to an Ub conjugating enzyme (Ubc or E2), again through a thiol-ester linkage. E3s are primarily responsible for providing specificity to Ub conjugation, interacting with E2 and substrate to facilitate the formation of isopeptide bonds between the C terminus of Ub and lysines on a target protein or on the last Ub of a protein-bound multi-Ub chain. Eukaryotic genomes encode a single or a few E1s, while there are substantially more E2s. The diversity and number of proteins regulated by ubiquitination suggest the existence of a large number of E3s. However, until recently, relatively few E3s were known. The discovery of E6-AP as an E3 responsible for human papilloma virus E6-dependent ubiquitination of p53 led to the identification of proteins containing a region of homology to the E6-AP carboxyl terminus (HECT domain), several of which have now been shown to be E3s. All these E3s include a RING finger either on the same polypeptide that recognizes substrate or as a distinct polypeptide in the context of a multi-subunit E3. The RING finger is a conserved domain that plays a critical role in ubiquitination. It is involved in the direct transfer of Ub from E2 to target lysines. The RING finger is also involved in the regulation of E3 activity, including interactions with E2s and substrates. RING finger proteins are involved in a variety of cellular processes, including cell cycle regulation, DNA repair, and apoptosis. They also play a role in the degradation of proteins and the regulation of protein trafficking. The RING finger is a key component of many E3s, and its function is essential for the ubiquitination process. The RING finger is involved in the regulation of E3 activity, including interactions with E2s and substrates. RING finger proteins are involved in a variety of cellular processes, including cell cycle regulation, DNA repair, and apoptosis. They also play a role in the degradation of proteins and the regulation of protein trafficking.RING finger proteins are essential mediators of ubiquitin ligase activity. Initially, the RING finger was thought to be involved only in protein dimerization, but recent studies have shown that RING finger proteins play critical roles in transferring ubiquitin (Ub) to both heterologous substrates and themselves. With hundreds of RING finger-containing proteins, the number of potential Ub protein ligases (E3s) has increased significantly. Ubiquitination begins with the formation of a thiol-ester linkage between the C terminus of Ub and the active site cysteine of the Ub activating enzyme (E1). Ub is then transferred to an Ub conjugating enzyme (Ubc or E2), again through a thiol-ester linkage. E3s are primarily responsible for providing specificity to Ub conjugation, interacting with E2 and substrate to facilitate the formation of isopeptide bonds between the C terminus of Ub and lysines on a target protein or on the last Ub of a protein-bound multi-Ub chain. Eukaryotic genomes encode a single or a few E1s, while there are substantially more E2s. The diversity and number of proteins regulated by ubiquitination suggest the existence of a large number of E3s. However, until recently, relatively few E3s were known. The discovery of E6-AP as an E3 responsible for human papilloma virus E6-dependent ubiquitination of p53 led to the identification of proteins containing a region of homology to the E6-AP carboxyl terminus (HECT domain), several of which have now been shown to be E3s. All these E3s include a RING finger either on the same polypeptide that recognizes substrate or as a distinct polypeptide in the context of a multi-subunit E3. The RING finger is a conserved domain that plays a critical role in ubiquitination. It is involved in the direct transfer of Ub from E2 to target lysines. The RING finger is also involved in the regulation of E3 activity, including interactions with E2s and substrates. RING finger proteins are involved in a variety of cellular processes, including cell cycle regulation, DNA repair, and apoptosis. They also play a role in the degradation of proteins and the regulation of protein trafficking. The RING finger is a key component of many E3s, and its function is essential for the ubiquitination process. The RING finger is involved in the regulation of E3 activity, including interactions with E2s and substrates. RING finger proteins are involved in a variety of cellular processes, including cell cycle regulation, DNA repair, and apoptosis. They also play a role in the degradation of proteins and the regulation of protein trafficking.