Researches on the area of apoptosis have achieved explosive progress over the past two decades. Two major regulating mechanisms of apoptosis have been uncovered in great details. One is called extrinsic apoptosis pathway, also known as the death-receptor pathway. The extrinsic pathway is usually activated from outside the cell by TNF and other cytokines with their corresponding death receptors, such as TNF receptor, Fas, and TRAIL. The other is named intrinsic apoptosis pathway, also called as the mitochondrial pathway. The intrinsic pathway is activated responding to cell stresses or intracellular damage, such as DNA damage.