Bad is a member of the Bcl-2 family proteins. Within the Bcl-2 homology domains 1 and 2 (BH1 and BH2)，Bad shows significant homology to Bcl-2 and Bcl-x. Bcl-2 is known to block several apoptosis signals and is considered to be a central downstream cell death repressor. Bcl-XL represses apoptosis，but its short form，Bcl-XS，promotes cell death. Bax is known to homodimerize as well as heterodimerize with Bcl-2. An excess concentration of Bax opposes the ability of Bcl-2 to repress cell death. Bad can selectively dimerize with Bcl-xL and Bcl-2，but not with Bax，Bcl-xS，Mcl-1，A1，or itself. In mammalian cells，Bad binds more strongly to Bcl-xL than Bcl-2，which may explain why Bad reverse the death repressor activity of Bcl-xL，but not that of Bcl-2. The formation of the Bad-Bcl-xL heterodimer displaces Bax and restores favorable conditions for apoptosis.
