Dominant regulator of apoptotic cell death. The long form displays cell death repressor activity, whereas the short isoform promotes apoptosis. Also acts as a regulator of G2 checkpoint and progression to cytokinesis during mitosis (By similarity). After neuronal stimulation, translocates from cytosol to synaptic vesicle and mitochondrion membrane in a calmodulin-dependent manner. Highest expression in organs with lymphoid development. The BH4 motif seems to be involved in the anti-apoptotic function. Intact BH1 and BH2 motifs are required for anti-apoptotic activity (By similarity). Belongs to the Bcl-2 family.