Fig. 3. Genomic constructs analysed for NCE activity by transgenesis in the mouse and electroporation in the chick neural tube. The -31/+7 Krox20/lacZ transgenic construct containing a lacZ in-frame insertion in Krox20 is indicated at the top. Restriction enzymes used to clone subfragments are shown. Distances are in kb and indicate the position relative to the start site of transcription of Krox20. Scale bars relevant to each subregion are indicated on the left. With the exception of constructs #1 and #2 which contain the Krox20/lacZ reporter, all subfragments were cloned into the ß-globin/lacZ promoter/reporter vector. Fragment #6 was cloned in both the sense (S) and antisense orientations (AS). K1/K2, Krox20 binding sites (open circle, wild type; X, mutated). For details of the Krox20 binding sites, their mutation, the regions deleted in constructs #19-23 and the sequences multimerised in construct #24 see Fig. 5. In mouse transgenic experiments, embryos were analysed at 9.5 dpc for ß-galactosidase activity. Those carrying construct #1 were analysed at F₁ and all others were analysed in `transient' transgenesis. Ratios correspond to the number of ß-galactosidase positive/total number of transgenic embryos. Unless indicated in brackets, mouse embryos were scored for expression in the r5 neural crest stream (NC). Embryos with expression in r5 were among those that showed expression in the neural crest. Chick embryos were scored at Hamburger and Hamilton stages 13-14 for ß-galactosidase activity in r3, r5 and the r5 neural crest stream (r3/r5+NC), unless indicated in brackets. NC, expression restricted to the r5 neural crest stream; CNC, expression throughout the cranial neural crest; +, constructs for which at least 60% of the electroporated embryos gave the indicated profile; -, those that were essentially negative in these tissues in at least two sets of five electroporated embryos; weak, when the levels of ß-galactosidase activity detected were considerably lower than for the controls; ND, not determined.

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