Fig. 1.

Construction of an inducible, cre lineage-tracing allele under the control of Tbx6 regulatory elements. A multipurpose targeting vector containing homology to the endogenous Tbx6 locus, an inducible cre gene, cre-ERT2, and a kanamycin/neomycin selection cassette surrounded by frt sites was targeted to a BAC containing the Tbx6 locus. Following homologous recombination, the targeted BAC was injected into the pronuclei of mouse zygotes to produce random-insertion transgenic mice. These were bred with mice containing a Flpe transgene to eliminate the selection cassette, resulting in a Tbx6 lineage-tracer allele inducible by tamoxifen.