Cortical neuronal cultures were prepared as described previously [38] from E16.5 littermate embryos obtained from timedpregnant heterozygous breedings and tissues not used for culture were genotyped. Cultures were maintained at 37uC under 5% CO 2 and half of the culture media was exchanged every 4-5 days. At day 10 in vitro, 5 mM camptothecin was added to the media to induce caspase activation [39] , and cells were harvested after 30 h of treatment by scraping in ice-cold PBS supplemented with protease inhibitors (4.2 mM Pefabloc and 'Complete' protease inhibitor cocktail (Roche)). Cells were pelleted by centrifugation and stored at -80uC until lysis. Cell pellets were lysed in 50 mM HEPES pH 7.4, 100 mM NaCl, 1% Igepal, 1 mM EDTA and 10% glycerol with 4.2 mM Pefabloc and 'Complete' protease inhibitor cocktail (Roche) on ice, and protein concentrations were determined in the cleared lysates after centrifugation.