Fig. 4. Search for enhancers of Otx2 expression in ectoderm at 3' downstream. (A) The 3' BAC #2/lacZ transgene and the 14 fragments assayed for enhancer activity. (B) ß-Gal expression driven by the 3' BAC #2/lacZ transgene (a), by #17 in nasal placode (b), by #20 in nasal cavity (c) and by #29 in forebrain and midbrain (d-i) at indicated stages. Open arrows, ß-gal expression in eyes; double arrows, ß-gal expression in nasal placode or cavity; arrowheads, the position of the isthmus. The activity of the 1.8 kb promoter in cephalic mesenchyme (indicated by an arrow in b) is suppressed by the 3' BAC #2 (a) and the #29 fragment (f,h); by contrast, promoter activity in anterior mesendoderm is not (an arrow in d). Open arrowheads in d indicate weak ß-gal expression in the future mid/hindbrain boundary region. With the exception of the nasal cavity, expression in c is ectopic. In lateral views (a-c,e-h), anterior is towards the left; (d,i), frontal views. Scale bars: 400 µm.