The E. coli lacZ gene was inserted, with translational reading frames aligned, into the first exon of the mouse Hoxd11 gene in a construct containing the entire Hoxd11 transcription unit and its known cis-acting 5'and 3' regulatory sequences. This fusion gene, followed by a loxP-flanked PGK-neo selection cassette, was targeted to the intergenic region between the transcription initiation sites of Evx2 and Hoxd13, at the 5' extremity of the Hoxd complex.