(a) HH-12 chicken embryos were electroporated with β-cateninS33Y in PiggyBac expression system vectors and left to develop for 7 or 15 days post electroporation (DPE). (b) 4′,6-diamidino-2-phenylindole (DAPI) and hematoxylin and eosin (H&E) staining of transverse brainstem sections at 7 DPE; scale bar, 150 μm. (c) Higher-magnification images of brainstem sections showing two examples of the aberrant accumulation of cells stained with antibodies against GFP (green, transfection) and proliferating cell nuclear antigen (PCNA) (red, mitosis marker). (d) Similar sections as in c were stained with phalloidin (red, labels F-actin) and antibodies against GFP (green, transfection) and Sox2 (blue, progenitor marker). (e) DAPI staining of 15 DPE transverse brainstem sections in which the floor of the IV ventricle and the mass are bounded by a dotted line. The right lower panels show amplifications of the two areas indicated by dotted squares in the upper panel that are labelled 1 and 2. Sections were stained with DAPI (blue) and antibodies against GFP (green, transfection) and Pax6 (red, marker of the external germinative layer of the cerebellum). Scale bar, 150 μm.