Suppresses apoptosis in a variety of cell systems including factor-dependent lymphohematopoietic and neural cells. Regulates cell death by controlling the mitochondrial membrane permeability. Appears to function in a feedback loop system with caspases. Inhibits caspase activity either by preventing the release of cytochrome c from the mitochondria and/or by binding to the apoptosis-activating factor (APAF-1). Forms homodimers, and heterodimers with BAX, BAD, BAK and Bcl-X(L). Heterodimerization with BAX requires intact BH1 and BH2 motifs, and is necessary for anti-apoptotic activity (By similarity). Also interacts with APAF1 and RAF-1 (By similarity). In adult chicken expressed, in thymus, spleen, kidney, heart, ovary and brain, with the highest levels in the thymus. In the embryo, highly levels expressed in all tissues with high levels in the bursa of Fabricius. The BH4 motif is required for anti-apoptotic activity and for interaction with RAF-1. Belongs to the Bcl-2 family.