Fig. 1. Identification of chick genomic regions containing Krox20 cis-regulatory elements. (A) Schematic representation of the chick Krox20 locus, BAC clones and subfragments tested by mouse transgenesis. Distances are relative to the start site of transcription of the Krox20 gene. The BACs (constructs 1 and 2) and the 65 kb fragment (construct 4) were injected alone and transgenic embryos were analysed by in situ hybridization with a chick Krox20 probe. The 42 kb and 30 kb fragments (constructs 3 and 5), which do not carry the Krox20 gene, were co-injected with a reporter fusion gene, Krox20/lacZ, and transgenic embryos were analysed for ß-galactosidase activity by X-gal staining. For each construct, the table indicates the number of E8.5 transgenic embryos obtained (n) and the number of embryos positive in r3 and r5 (r3/r5), or in the r3 to r5 region (r3-r5). (B-E) Dorsal views of embryos transgenic for the indicated constructs and analysed as indicated in A. Embryos are rostral side upwards. r, rhombomere; ov, otic vesicle.