GTPases like Ras, Rho, cdc42Hs, and Rac modulate a multitude of cellular functions like cytoskeletal architecture, growth, motility, and gene expression. The activity of the GTP-binding proteins is regulated by factors that accelerate GTP-binding (GAPs) and proteins that enhance the rate of GTP hydrolysis. mDia1, also known as p140mDia and Drf1, is the mammalian homologue of Drosophila's diaphanous, a protein essential for cytokinesis. The 1255 amino acid mDia1 is widely expressed, with a Rho binding domain at the NH2-terminal region, a central polyproline region, and an FH2 domain. This protein also shares homology with the yeast Bn1p essential for budding and the mouse formin necessary for proper limb development. mDia1 binds to GTP-Rho and to the actin-binding protein profilin-all three proteins are co-localized at the lamellipodia in cultured cells. The overexpression of mDia1 promoted the formation of actin filaments, implicating this protein in cell motility events regulated by Rho. mDia1 was also found consistently mutated in familial deafness.