Bcl-2 family proteins are a group of proteins homologous to the Bcl-2 protein and characterized by containing at least one of four conserved Bcl-2 homology (BH) domains (BH1, BH2, BH3 and BH4). Bcl-2 family proteins, consisting of pro-apoptotic and anti-apoptotic molecules, can be classified into the following three subfamilies according to sequence homology within four BH domains: (1) a subfamily shares sequence homology within all four BH domains, such as Bcl-2, Bcl-XL and Bcl-w which are anti-apoptotic; (2) a subfamily shares sequence homology within BH1, BH2 and BH4, such as Bax and Bak which are pro-apoptotic; (3) a subfamily shares sequence homology only within BH3, such as Bik and Bid which are pro-apoptotic.