An energy-dependent, programmed form of cell death that is characterized by phenotypic markers such as increased levels of reactive oxygen species (often detected via DHE staining), externalization of phosphatidylserine (measured by annexin V staining), and DNA fragmentation (detected using the TUNEL assay). Other processes observed during apoptosis include chromosome condensation and breakdown of mitochondria, including loss of membrane potential and release of cytochrome c. Cells undergoing apoptosis do not lose plasma membrane integrity (measured by propidium iodide staining) until late in the process.