Programmed cell death (PCD) refers to an intracellularly regulated shut down of a cell. Cells die via PCD in response to toxic stimuli, abnormal nucleic acid structures, intracellular detection of foreign material, cell-to-cell signaling during development or immune activity, or other possible events that would be disadvantageous to the host. A physiological important programmed cell death mechanism is apoptosis, which is tightly controlled by caspases.