A cre/ERT2-polyA cassette and loxP-flanked neomycin selection cassette were inserted into the Eomes locus at the translational start site in exon 1 by homologous recombination. This resulted in deletion of about 500 bp of the coding region of exon 1 (creating a null allele). The neo selection cassette was deleted in the final knockin allele. In situ hybridization demonstrated that embryonic expression of cre/ERT2 recapitulated the spatiotemporal expression pattern of endogenous Eomes.