The TBC domain is an evolutionarily conserved protein domain found in all eukaryotes. It is approximately 180 to 200 amino acids long. The domain is named for its initial discovery in the proteins Tre-2, Bub2, and Cdc16. TBC family members act as GTPase-activating proteins (GAPs) for small GTPases in regulating the cell cycle. For example, Rab activity is modulated in part by GAPs, and many RabGAPs share a Tre2/Bub2/Cdc16(TBC)-domain architecture, suggesting that TBC domain-containing proteins may behave similarly.