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. A construct containing this fusion gene followed by an oppositely oriented 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, with Hoxd11/lacZ in the same transcriptional orientation as Hoxd13. The selection cassette was subsequently deleted by Cre recombinase-mediated recombination, leaving a single loxP site downstream/3' of the Hoxd/lacZ fusion gene.