The remodelling of the actin cytoskeleton is mediated by a number of factors that affect the formation and dynamics of actin filaments. Among these are the formins, which promote the nucleation and elongation of actin filaments. In humans, there are 15 formins and additional splicing isoforms; they all contain a FH2 dimerisation domain, which has been shown to promote the initial actin filament nucleation and the barbed-end elongation of actin filaments. However, the biochemical mechanisms of formin-mediated actin polymerisation have thus far only been described in vitro for formin constructs containing only the FH1 and FH2 domains owing to difficulties in stably expressing the full proteins. André Schönichen, Matthias Geyer and colleagues (p. 1891) now investigate actin polymerisation mediated by the formin FHOD1, which has been shown to induce the formation of actin stress fibers. Using in vitro actin polymerisation assays with full-length and different deletion constructs of FHOD1, they find that FHOD1 lacks the ability to nucleate and elongate actin filaments, but instead acts as an actin-bundling factor that caps the barbed end of filaments. These results are also confirmed in cells that constitutively express FHOD1. On the basis of this data, the authors propose that FHOD1 stabilises actin filaments by protecting their barbed ends from depolymerisation with its dimeric FH2 domain, whereas the region N-terminal to the FH1 domain mediates actin bundling by simultaneously binding to the sides of adjacent actin filaments.