A sequence encoding cre together with an FRT-flanked kanamycin cassette was inserted into a BAC (RP24-145C8) into exon 1 of the Flt3 locus by ET cloning. Negative selection to identify properly targeted colonies was performed by transfection with an Flp recombinase-containing plasmid and selecting for kanamycin-sensitive colonies. Recombination was confirmed by sequencing and properly targeted BACs were injected into C57BL/6 pronuclei. Multiple lines were generated but no line numbers were specified.