(A) BrdU uptake in E9.5 embryos as a measure of cell proliferation. Nuclei that incorporated BrdU are stained brown. (B) TUNEL staining in E9.5 embryos to detect apoptosis. Exposed DNA ends characteristic of apoptotic cells were labeled brown. In both A and B sets of pictures, (a, b, c) represent the forebrain and (d, e, f) represent somites. +/+ indicates Mrad9b+/+; +/− indicates Mrad9b+/−; −/− indicates Mrad9b−/−. The three embryos are from the same litter. m: mesenchyme, ne: neural epithelium. Arrowhead indicates apoptotic cell(s). Magnification ×40.