The ability of cell surface integrins to link the extracellular matrix(ECM) to the actin cytoskeleton is central to cell motility: if an ECM-anchored integrin is attached to the cytoskeleton, the cytoskeleton can pull on it and so drive the cell forward. Cytoskeletal attachment appears to require integrin clustering, but how big must each cluster be? Harold Erickson and co-workers have approached this question by examining the number of integrin molecules needed to translocate fibronectin-coated beads across the surface of a cell at constant velocity — an assay for cytoskeletal attachment (see p. 2581). They find that beads coated with fibronectin constructs containing three or five cell-adhesion domains, which should produce clusters of three and five integrins, respectively, bind strongly to and translocate across the cell surface. Fibronectin constructs containing one or two adhesion domains (which would form only integrin monomers/dimers) by contrast exhibit only brief binding and random movement. A cluster of at least three integrins thus seems to be required for productive cytoskeletal interactions — which is an interesting contrast to the paradigm of receptor dimerization in signalling.