Fig. 4. svp expression in the proneural cluster. (A) BrdU incorporation (purple) in the tubules of stage 12 embryos expressing svp-lacZ (green) demonstrates that the cells in S phase also express svp (overlay; white). (B,C) Kr staining (purple) marks the tubule cells and is more strongly expressed in the tip mother cell at early stage 11(arrowhead, B). Slightly later, Kr expression is lost from cells surrounding the tip mother cell (C). svp-lacZ is expressed in wild-type embryos before the tip mother cell divides (B, in C the tip mother cell is in anaphase) and therefore before EGF receptor-driven cell divisions. (D) In AS-C^–/– [Df(svr)] embryos, there is no PNC and svp-lacZ is not expressed during stage 11 in the tubules. (E) In N^55e11 embryos, supernumerary TCs, which develop from the PNC, express high levels of Kr and svp-lacZ, while other tubule cells express neither. (F) In embryos mutant for EGFR (top^CO), svp-lacZ is still expressed in a cluster of tubule cells at stage 11, as in wild type (B).