Figure 4.

lacZ expression in transgenic embryos. Representative embryos generated from ES clone 91 and its targeted derivatives and stained for β-galactosidase activity are shown in (a)–(d) (day 10.5 of development) and (e) (day 16). Arrows indicate sites of ectopic expression. Embryos generated from ES clone 710 and its targeted derivatives are shown in (f)–(i) (day 10.5) and (j) (day 16). No ectopic expression was observed. lacZ expression was directed by the Oct1.9 promoter (a) the Oct8.5 promoter (f) the Hox2.6 promoter (b and g), the Myf5 promoter (c and h) and the thyroglobulin promoter (d, e, i and j). Embryos in (a)–(e) were generated by tetraploid rescue and in (f)–(j) by diploid aggregation. Panels (e) and (j) are thyroid glands following removal of overlying tissue in day 16 embryos.