Exons 3-5 (the entire coding region) were replaced with a reverse tetracycline controlled transactivator (rtTA) gene and a loxP-flanked Neo cassette, placing the start codon of the rtTA at the position of the endogenous start codon in exon 3, via homologous recombination. The Neo cassette was removed by crossing to mice expressing germline Cre-recombinase. Crossing to mice expressing lacZ under the control of the tetracycline inducible promoter and then treating with doxycycline resulted in X-gal staining (indicating rtTA activity) that recapitulated Sox10 expression.