<p>Active cell suicide (apoptosis) is induced by events such as growth factor withdrawal and toxins. It is controlled by regulators, which have either an inhibitory effect on programmed cell death (anti-apoptotic) or block the protective effect of inhibitors (pro-apoptotic) [<cite idref="PUB00001030"/>,<cite idref="PUB00001926"/>]. Many viruses have found a way of countering defensive apoptosis by encoding their own anti-apoptosis genes preventing their target-cells from dying too soon.</p><p> All proteins belonging to the Bcl-2 family [<cite idref="PUB00000008"/>] contain either a BH1, BH2, BH3, or BH4 domain. All anti-apoptotic proteins contain BH1 and BH2 domains, some of them contain an additional N-terminal BH4 domain (Bcl-2, Bcl-x(L), Bcl-w), which is never seen in pro-apoptotic proteins, except for Bcl-x(S). On the other hand, all pro-apoptotic proteins contain a BH3 domain (except for Bad) necessary for dimerisation with other proteins of Bcl-2 family and crucial for their killing activity, some of them also contain BH1 and BH2 domains (Bax, Bak). The BH3 domain is also present in some anti-apoptoticprotein, such as Bcl-2 or Bcl-x(L). This profile is found in all these proteins, as well as E1B 19K protein (small t-antigen), which inhibits E1A inducedapoptosis and hence prolongs the viability of the host cell.</p>