Fig. 4. Modification of rhombomere identity of r3 cells in Krox20 mutant embryos. (A-C) Flat-mounted hindbrains of 8 s Krox20+/+ (A), Krox20^lacZ/+ (B), or Krox20^lacZ/lacZ (C) embryos carrying the AP transgene expressed in r2 (r2AP), stained for AP activity and processed for Hoxb1 ISH (both purple). (D) Hindbrain of an 8 s Krox20^lacZ/lacZ whole embryo labelled with X-gal for 12 hours to show the presence of r3 cells at this stage. (E) r2-r3 region of the flat-mounted hindbrain of a Krox20+/+ r2AP embryo at the 10 s stage, processed for Krox20 protein immunochemistry (orange) and AP activity (purple). (F) The r2/r4 region of the flat-mounted hindbrain of an 8 s Krox20^lacZ/lacZ r2AP embryo, stained for X-gal (blue) and AP activity (purple). The inset shows a high magnification of three cells, one of which is a double stained cell. (G) The r2/r4 region of the flat-mounted hindbrain of an 8 s Krox20^lacZ/lacZ embryo stained with X-gal (blue) and processed for Hoxb1 ISH (brown). (H) r3/r5 region of the flat-mounted hindbrain of a 9.5 dpc Krox20Cre/lacZ R26R embryo stained with X-gal (blue) and processed for Hoxb1 ISH (purple). The arrowheads in F-H point to cells labelled by both markers. Rostral is towards the top.