e36 Nucleic Acids Research, 2005, Vol. 33, No. 4 gene, thus creating tight arabinose-inducible expression for any gene of interest. This is done by ﬁrst replacing the Cre/ Flpe ORF with galK and select for Gal+ recombinants. The galK cassette is then replaced by the ORF of the gene of interest by DOG selection for Gal recombinants. Finally, the strains described here should be very useful for genetic manipulation not only of BACs, but also the E.coli genome itself. All recombineering reagents discussed in this work are freely available upon request. To obtain these materials, please follow the directions listed on our website (http:// recombineering.ncifcrf.gov). Detailed protocols for recom- bineering, including galK selection, can also be downloaded directly from our website. ACKNOWLEDGEMENTS We thank Allen Kane and Carolyn Whistler from Scientific Publications, Graphics & Media, NCI-Frederick, for help with illustrations and figure preparation. 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