RHOBTB1 (Rho-related BTB domain-containing protein 1) and RHOBTB3 (Rho-related BTB domain-containing protein 3) each contain two BTB (POZ) domains and belong to the RhoBTB subfamily of Rho GTPases. Members of the RhoBTB subfamily are evolutionarily conserved and are characterized by a proline-rich region, a GTPase domain and two tandem BTB repeats. While both RHOBTB1 and RHOBTB3 are expressed ubiquitously, RHOBTB1 is found at high levels in placenta, stomach, testis, kidney and skeletal muscle, whereas RHOBTB3 is found at high levels in neural and cardiac tissues. RHOBTB1 is thought to play a role in GTPase-mediated signaling and may participate in organization of the Actin filament system. Additionally, RHOBTB1 expression is decreased in head and neck carcinomas, suggesting a possible role for RHOBTB1 as a tumor suppressor.