Apoptosis regulator, Bcl-X protein <p>Apoptosis, or programmed cell death (PCD), is a common and evolutionarily conserved property of all metazoans [<cite idref="PUB00017281"/>]. In many biological processes, apoptosis is required to eliminate supernumerary or dangerous (such as pre-cancerous) cells and to promote normal development. Dysregulation of apoptosis can, therefore, contribute to the development of many major diseases including cancer, autoimmunity and neurodegenerative disorders. In most cases, proteins of the caspase family execute the genetic programme that leads to cell death.</p>Bcl-X is a dominant regulator of programmed cell death in mammalian cells. The long form (Bcl-X(L)) displays cell death repressor activity, but the short isoform (Bcl-X(S)) and the b-isoform (Bcl-Xb) promote cell death. Bcl-X(L), Bcl-X(S) and Bcl-Xb are three isoforms derived by alternative RNA splicing. Bcl-X(S) forms heterodimers with Bcl-2. Homologues of Bcl-X include the rat Bax and mouse Bak proteins which also influence apoptosis.