Bax is a 21kD protein that was identified as a protein that associates with Bcl-2, a repressor of apoptosis. Bax is a member of the bcl-2 family of proteins and contains Bcl-2 homology regions (BH-) 1, 2 and 3. Alternative splicing of Bax mRNA results in a 21kD membrane (alpha) and two forms of cytosolic proteins (beta and gama). Unlike Bcl-2, Bax initiates apoptosis when it homodimerizes or when it fails to form heterodimers with Bcl-2. Overexpression of Bax increased the rate of apoptosis caused by cytokine deprivation in an IL-3-dependent manner. Bax expression is stimulated by transcriptional activator p53, a tumor suppressor protein that induces apoptosis caused by damage to DNA.