Programmed cell death or apoptosis is required for the patterning and 
development of multicellular organisms. However, apoptosis is a difficult 
process to measure because the dead cells are rapidly degraded by their 
neighbors within a few hours. The post-caspase activation events that determine 
whether a cell will undergo apoptosis remain elusive. Here we report that 
apoptosis-specific nuclear events that occur before DNA fragmentation can be 
distinguished by monitoring the histone H1 status. In both mammals and 
Drosophila, dying cells failed to be immunolabeled with an anti-H1 monoclonal 
antibody, AE-4. Real-time imaging of caspase activation and H1 dynamics in 
mammalian neural cells revealed that H1 changed its location in the nucleus 
after caspase activation. In addition, the timing of this re-localization was 
largely dependent on the apoptotic stimulus used. From the staining patterns of 
AE-4 and anti-active caspase-3 antibodies, cells undergoing the transition from 
caspase activation to the apoptotic H1 change could be identified as H1-positive 
caspase-activated cells, providing a novel criterion for early apoptosis and 
making it possible to characterize caspase-activated cells in tissues. On the 
basis of these staining patterns, we found that many olfactory sensory neurons 
in the developing mouse olfactory epithelium showed sustained caspase activity 
without the H1 change, suggesting a unique caspase function in these neurons.