Vaccinia virus DNA polymerase (VVpol) encodes a 3'-to-5' proofreading exonuclease that can degrade the ends of duplex DNA and expose single-stranded DNA tails. The reaction plays a critical role in promoting virus recombination in vivo because single-strand annealing reactions can then fuse molecules sharing complementary tails into recombinant precursors called joint molecules. We have shown that this reaction can also occur in vitro, providing a simple method for the directional cloning of PCR products into any vector of interest. A commercial form of this recombineering technology called In-Fusion(®) that facilitates high-throughput directional cloning of PCR products has been commercialized by Clontech. To effect the in vitro cloning reaction, PCR products are prepared using primers that add 16-18 bp of sequence to each end of the PCR amplicon that are homologous to the two ends of a linearized vector. The linearized vector and PCR products are coincubated with VVpol, which exposes the complementary ends and promotes joint molecule formation. Vaccinia virus single-stranded DNA binding protein can be added to enhance this reaction, although it is not an essential component. The resulting joint molecules are used to transform E. coli, which convert these noncovalently joined molecules into stable recombinants. We illustrate how this technology works by using, as an example, the cloning of the vaccinia N2L gene into the vector pETBlue-2.