Whereas nematodes contain a single BH3-only protein, EGL-1, mammals express at least eight, including Bad, in a cell type- and stress-specific manner. The pro-apoptotic activities of these proteins are tightly regulated by diverse transcriptional and post-transcriptional mechanisms. Two BH3-only proteins, Puma and Noxa (see Figure 21-40), are transcriptionally induced by the p53 protein (see Figure 19-29). This interaction is part of the checkpoint pathway by which unrepaired damage to DNA can induce apoptosis; thus the loss of p53 seen in many cancers allows cells to live with severe DNA damage (see Figure 24-27). Another BH3-only protein, Bim, is normally sequestered by the microtubule cytoskeleton by binding to a dynein light chain (see Figure 18-24). Detachment of cells from their substratum disrupts integrin signaling, rearranges the cytoskeleton, and leads to release of Bim. Both Puma and Bim bind directly to Bak and Bax as well as to Bcl-2, releasing Bak and Bax from Bcl-2 and allowing formation of pores and holes in the outer mitochondrial membrane (see Figure 21-40). Thus apoptosis of mammalian cells is regulated by a careful balance of activities of anti-apoptotic proteins such as Bcl-2 and Bcl-xL and multiple pro-apoptotic BH3-only proteins.