CASP3 and microcephaly: Chmp1a null embryos had no detectable increase in cleaved caspase-3-positive apoptotic cells (Figures S4C and S4D) and intact epithelial structure with normal localization of key proteins such as atypical protein kinase C (aPKC) and β-catenin (Figure S4E), suggesting that the microcephaly and cortical thinning result from decreased progenitor proliferation.