The cytoskeleton is made up of arrays of actin filaments that are arranged into widely different structures — parallel arrays that mediate muscle contraction, networks of branched filaments at the leading edge of migratory cells, and parallel bundles required to pinch cells apart at the end of cell division. The critical juncture of filament growth is the process of nucleation, in which two or three actin monomers come together to form a “seed” for actin polymerization. Afterwards, the assembly of filamentous actin takes off. Because nucleation is the slowest step in filament assembly, it is the nodal point at which many signals converge. “When a cell wants to make some actin, one way to do it is to promote nucleation and shorten the lag before filament assembly,” said David Pellman, Harvard Medical School professor of pediatrics at the Dana-Farber Cancer Institute. He and his colleagues discovered a mechanism for nucleating actin filaments that is mediated by conserved formin proteins in conjunction with another conserved protein, profilin. The research, a collaboration with the lab of Bruce Goode at Brandeis University, was published in Nature Cell Biology in August 2002.