Taking the advantage of single-molecule imaging, our recent study has revealed surprisingly long processive movement of a Formin protein, mDial, surfing along with the growing end of actin filaments in living cells. This finding provides direct evidence for the ability of Formins to function as processive cappers that has been postulated from several lines of evidence in biochemical studies. With nucleating filaments from the profilin-actin pool, Formins may effectively generate long actin filaments, and contribute to the generation of the specific actin-based structures, that is, the contractile ring in cytokinesis, actin stress fibers in animal cells, and yeast actin cables. Furthermore, Formins have the potential to function as actin polymerization-driven molecular motors. Although much remains to be tested about the role of this novel molecular mobilization mechanism, cells might utilize actin polymerization energy for cell shape change and/or trafficking via Formin motors. (C) 2004 Elsevier Inc. All rights reserved.