A double membrane surrounds some nuclear fragments, but others are not membrane bound. Apoptotic cells express markers (e.g., phosphatidylserine or sugars) on their surface for recognition by phagocytes. The final phase of apoptosis in vivo is phagocytosis of cellular debris (Fig. 2). Naturally-occurring PCD (Fig. 2) is the 'gold standard' for apoptosis to which other forms of cell death should be compared for classification (8,9).