Expressed mainly in breast, thymus, prostate, fetal liver, colon, placenta, pancreas, small intestine, spinal cord, kidney, and bone marrow and to a lesser extent in many other tissues. Isoform 2 is primarily expressed in skeletal muscle. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the Bcl-2 family.