Many debilitating diseases, including neurodegenerative diseases, involve 
apoptosis. Several methods have been developed for visualizing apoptotic cells 
in vitro or in fixed tissues, but few tools are available for visualizing 
apoptotic cells in live animals. Here we describe a genetically encoded 
fluorescent reporter protein that labels apoptotic cells in live zebrafish 
embryos. During apoptosis, the phospholipid phosphatidylserine (PS) is exposed 
on the outer leaflet of the plasma membrane. The calcium-dependent protein 
Annexin V (A5) binds PS with high affinity, and biochemically purified, 
fluorescently labeled A5 probes have been widely used to detect apoptosis in 
vitro. Here we show that secreted A5 fused to yellow fluorescent protein 
specifically labels apoptotic cells in living zebrafish. We use this fluorescent 
probe to characterize patterns of apoptosis in living zebrafish larvae and to 
visualize neuronal cell death at single-cell resolution in vivo.