Fig. 5. Identification of a promoter fragment responsive to Hoxa2. (A) The most proximal 900 bp of the Six2 promoter (-893; +37) are sufficient to drive lacZ expression in the proximal area of the first branchial arch, similarly to Six2 endogenous expression. (B) In the absence of Hoxa2, an equivalent staining to the first arch was observed in the second branchial arch (arrowheads) and in the mesenchyme proximal to the branchial arches (arrows), reproducing the Six2 expression pattern observed in Hoxa2 mutant embryos (see Fig. 1; white asterisk: first cleft). Higher expression of the transgene was also observed in the caudal area of the second branchial arch, where Six2 is not differentially regulated (white arrow). I, first branchial arch; II, second branchial arch.