Bok is a proapoptotic member in the Bcl-2 family and the expression of Bok is regulated by cellular stresses. In the present study, we have isolated and characterized a natural antisense transcript of Bok, BOKAS. The BOKAS gene consists of 2 exons, that expresses as a non-protein coding transcript. Both BOKAS and Bok are transcribed from the same locus but in opposite orientations. The mRNA expression of BOKAS was only detected in testis and certain cancer tissues but not in other normal adult tissues examined. Overexpression of BOKAS alone exhibited no significant anti- or pro-apoptotic activity but it was able to inhibit Bok-induced apoptosis in HeLa cells. Our results suggest that BOKAS may function specifically in the human testis, where it serves as an antisense molecule to regulate Bok-induced apoptosis. In addition, natural antisense transcripts were identified for BAD and BCL2L12.