Single-molecule studies reveal how the DNA-repair protein RecA overcomes competition from another protein to bind to single-stranded DNA, and how other mediator proteins assist in this process. See Letter p.274 An early step in the repair of DNA lesions or stalled replication forks by homologous recombination involves the pairing of homologous DNAs by the strand-exchange protein RecA. RecA from the bacterium Escherichia coli nucleates binding on single-stranded DNA (ssDNA) as a small cluster of subunits, after which it extends to form a protein–DNA filament. Until now, it has not been possible to visualize its assembly on ssDNA because of the difficulty of creating ssDNA that can be readily used in single-molecule experiments. But now Stephen Kowalczykowski and colleagues have characterized the nucleation and extension of this filament by single-molecule microscopy. The data indicate why 'mediator' proteins might be necessary to facilitate loading and growth, predictions that are confirmed by adding the bacterial RecFOR complex to the reaction.