FIGURE 21-39 Structures of members of the Bcl-2 family of proteins. The Bcl-2 family, which comprises proteins that contain functional Bcl-2 homology domains (BH1–4), can be divided into three classes. All of the pro-apoptotic and anti-apoptotic proteins, but only some of the BH3-only proteins, contain a hydrophobic and presumably transmembrane (TM) domain that may function to anchor the protein in the outer mitochondrial membrane. See M. Giam et al., 2009, Oncogene 27:S128.