PUMA (Bcl-2 binding component 3, JFY1, PUMA/JFY1) is a mitochondrial pro-apoptotic Bcl-2 homology domain (BH3)-only protein that induces rapid apoptosis through a Bax- and mitochondria-dependent pathway. The PUMA gene encodes four proteins originating from different splice variants of the same transcript: PUMAalpha, beta, gamma and delta. Both PUMAalpha and PUMAbeta contain a BH3 domain, while PUMAgamma and PUMAdelta lack this domain. The BH3 domain is essential for binding of PUMAalpha and PUMAbeta to Bcl-2 or Bcl-xL. PUMA is an initiator of gamma -radiation apoptosis and glucocorticoid-induced apoptosis in lymphoid cells in vivo. Bcl-2 family members generally regulate apoptosis and transmit death signals to mitochondria. Members of this family include both pro- and anti-apoptotic proteins that share homologous sequences known as Bcl-2 homology domains (BH1-4). The BH3 proteins, BID, NOXA, PUMA, NBK, Bim and Bad, are all pro-apoptotic and share sequence homology within the amphipathic alpha-helical BH3 region.