The antiapoptotic proteins, such as Bcl-xL in mammals and CED-9 in nematodes, interact with proapoptotic members by recognizing their BH3 domains. The specific recognition was revealed by the structure of Bcl-xL bound to a BH3 peptide derived from Bak (Sattler et al. 1997), Bad (Petros et al. 2000), or Bim (Liu et al. 2003) (Figure 4d ). The BH3 peptide forms an amphipathic α helix and interacts with a deep hydrophobic groove on the surface of Bcl-xL. The binding of the BH3 domain is accompanied by a significant conformational change in Bcl-xL. These observations were further confirmed by the structure of CED-9 bound to an extended BH3 domain from EGL-1 (Yan et al. 2004a). Despite limited sequence similarity between Bcl-xL and CED-9, their structures exhibit identical features. The molecular recognition between CED-9 and EGL-1 closely parallels those between Bcl-xL and Bim (Figure 4e). Comparative analysis of these protein complexes will likely reveal some general code that governs the interaction among the Bcl-2 family of proteins.