A cre/ERT2 cassette with a floxed neo gene was inserted by RedE/T recombineering into a BAC containing the Cx3cr1 locus, replacing the coding exon of Cx3cr1. Properly targeted ES cells were aggregated with 8-cell embryos to generate chimeras. Chimeras were backcrossed to C57BL/6 mice. Tne neo was excised by crossing the animals to a cre deleter strain (Tg(Pgk1-cre)1Lni).