BID is a pro-apoptotic Bcl-2 protein containing only the BH3 domain. In response to apoptotic signaling, BID interacts with another Bcl-2 family protein, Bax, leading to the insertion of Bax into organelle membranes, primarily the outer mitochondrial membrane. The expression of BID is upregulated by the tumor suppressor p53, and BID has been shown to be involved in p53-mediated apoptosis.