Bok (Bcl-2-related ovarian killer) is a pro-apoptotic Bcl-2 family protein identified in the ovary based on its dimerization with the anti-apoptotic protein Mcl-1. In addition to the Bcl-2 homology (BH) domains 1 and 2 and the transmembrane sequence, Bok also has a BH3 domain believed to be important for dimerization with selective anti-apoptotic Bcl-2 proteins and cell killing. Bok interacts strongly with some (Mcl-1, BHRF1, and Bfl-1) but not other (Bcl-2, Bcl-xL, and Bcl-w) anti-apoptotic members. In addition, cell killing induced by Bok was suppressed following coexpression with Mcl-1 and BHRF1, but not with Bcl-2, further indicating that Bok heterodimerized only with selective anti-apoptotic Bcl-2 proteins. Bok was highly expressed in the ovary, testis and uterus.