A fundamental characteristic of multicellular organisms is the ability to activate a gene-encoded cell suicide program called programmed cell death (PCD). PCD serves many important functions in animal development and homeostasis, such as sculpting body structures, deleting unneeded structures, adjusting cell numbers, and eliminating abnormal, misplaced, nonfunctional, or harmful cells. The pathogenesis of many diseases, including cancer and neurodegenerative disorders, is attributed to the malfunctioning of PCD. Apoptosis, the most common form of PCD, is characterized by a conserved sequence of morphological, cellular, biochemical and molecular events. A key feature of apoptosis is the activation of a unique family of cysteine aspartyl proteases called caspases.
Caspases are synthesized as inactive proenzymes, which work together in a precisely controlled proteolytic cascade to activate themselves and one another. Downstream of this activational cascade, caspases cleave a variety of regulatory and structural proteins and important enzymes, targeting the cell for destruction by disassembling its contents. However, a critical mass of data indicates that apoptotic caspases are also involved in a variety of non-lethal cellular processes generally called CDPs (caspase-dependent non-lethal cellular processes). On the other hand, PCD can sometimes proceed in the absence of caspase activity, through alternative cell death pathways or ACDs.