Recombinant DNA clones containing large pieces of DNA are useful in the study of large genetic units, but these are difficult to make in most bacterial cloning vectors. A strategy is described that uses general and sitespecific recombination to construct large pieces of eukaryotic DNA from smaller cloned segments. The large clones are propagated on F factor-based plasmids in Escherichia coli. They can be easily modified to introduce mutations or rearrangements. These techniques were applied to the construction of large DNA segments from the bithorax complex of Drosophila.