Consistent with the results of Li et al. [31], we found particular brain malformations in our Ptdsr -/- mice. Exencephaly and hyperplastic brain phenotypes were observed at a low penetrance in Ptdsr-mutant mice (less then 4.5% of homozygotes), but these do not resemble to any extent the brain-overgrowth phenotypes of caspase- or Apaf1-knockout mice ([44], and references therein) in that we failed to identify any differences in the number or distribution of apoptotic cells or pyknotic cell clusters in the neuroepithelium of Ptdsr -/- and Ptdsr+/+ mice. Thus, reduced cell death or diminished clearance of apoptotic neural progenitor cells is unlikely to be the cause of the brain hyperplasia.