Bik/Nbk (Bcl-2-interacting killer/natural born killer) is a potent pro-apoptotic protein belonging to a group of Bcl-2 family members that includes Bad, Bid, Bim, Hrk, and Noxa, containing a BH3 domain but lacking other conserved domains, BH1 or BH2. Functionally, Bik is able to bind to and antagonize anti-apoptotic Bcl-2 family members including Bcl-2, Bcl-xL, and viral homologs E1B-19K and EBV-BHFR1. The BH3 domain of Bik is essential for its apoptotic activity and interaction with survival proteins. Phosphorylation of Bik is correlated with an increase in its pro-apoptotic activity.