Fig. 1. Activity of the core element of the Foxa2 node/notochord enhancer in transgenic mouse embryos. (A) Schematic representation of the mouse Foxa2 enhancer. The node/notochord enhancer is located upstream of the transcription initiation site (yellow square). CS3 (red circle) is essential for enhancer activity. A 27-bp DNA fragment straddling CS3 is the core element (CE) of the enhancer. (B-J) Distribution of ß-galactosidase activity in transgenic mouse embryos that express LacZ under the control of eight copies of the CE. Whole-mount staining of LS (B), EB (C), LHF (D,E), E8.5 (G) and E9.0 (H) stage embryos. Sagittal section of an LHF embryo crossing through the node (F), and cross-sections of an E8.5 embryo (I,F). Approximate positions of sections shown in panels I and F are indicated in panel G by i and j, respectively. ame, axial mesoendoderm; ec, ectoderm; en, endoderm; h, heart; hf, headfold; hm, head mesenchyme; m, mesoderm; mb, midbrain; n, node; nc, notochord; nf, neural fold; np, neural plate; ov, otic vesicle; pm, presomitic mesoderm; pp, prechordal plate; ps, primitive streak; s, somite; tb, tailbud.