Members of the Bcl-2 family of proteins interact to regulate programmed cell death, or apoptosis. Various homodimers and heterodimers formed by proteins in this family can either promote or inhibit apoptosis. Bcl-2 blocks cell death following a variety of stimuli and confers a death-sparing effect on certain hematopoietic cell lines following growth factor withdrawal. Additional apoptotic inhibitors in this family include Bcl-x, Bcl-w, Mcl-1, Bag-1 and A1. Pro-apoptotic members of this family include Bax, Bad, Bak, NBK (Bik), BID and Hrk. Hrk (for harakiri), designated DP5 or neuronal death protein in mouse and rat, contains a BH3 domain with high homology to other Bcl-2 family members but lacks the conserved BH1 and BH2 domains. Physical interaction of Hrk with Bcl-2 or Bcl-xL inhibits the apoptotic activity of Hrk.