The G2-M checkpoint or for post-irradiation induction of apoptosis. In this study, we addressed the localization pattern of the Drosophila 9-1-1 NVP-BKM120 supply complex and analyzed the importance of the localization pattern of DmRad9A during activation of the meiotic checkpoint. In this study, we showed that the Drosophila 9-1-1 proteins present an almost similar localization pattern as do their mammalian homologues. Both hRad1 and hHus1 are cytoplas mic, while hRad9 is a nuclear protein. In Drosophila the two alternatively spliced forms of DmRad9 are both localized to the nucleus, yet DmRad9A is a nuclear membrane-bound protein. Moreover, when all three proteins were co-over-expressed, DmRad9A and DmRad9B determined the localization of the two other proteins, namely DmRad1 and DmHus1. The same is also true in mammalian cells, where it was suggested that genotoxic stress induces the expression of hRad1, which in turn stabilizes hHus1. Once all proteins are present, hRad9 transports the complex into the nucleus. We have shown that DmRad9 possesses a NLS near the Cterminus of the protein. Much like its human counterpart, the DmRad9 NLS was found to be crucial for localization of the protein to the nucleus. Interestingly, the NLS of hRad9 and DmRad9 reside in the C-terminal regions of both proteins, no other similarity exists between these two proteins in this region. The first 274 amino acids of the human Rad9 protein show relatively high similarity to the first 268 amino acids of DmRad9. Moreover, both the human Rad9 NLS motif, which lies between amino acids 356 to 364, and the DmRad9 NLS motif, found between amino acids 300 to 302, are not conserved. Despite these differences, DmRad9A is localized to nuclear membrane when expressed in mammalian cells, suggesting that the mechanism by which the protein is targeted to the nuclear membrane is likely conserved. Previously, it had been shown that DmHus1 is involved in activation of a meiotic checkpoint. Moreover, as described in this study, the DmRad9A transcript is more abundant than is the DmRad9B transcript during oogenesis. Thus, the physiological function of DmRad9A nuclear membrane localization during activation of the meiotic checkpoint was studied. The Drosophila meiotic checkpoint was first revealed upon study of a class of mutant genes that required for the repair of recombinationinduced DSBs during Drosophila oogenesis. Mutations in these genes lead to activation of a meiotic checkpoint, leading to the appearance of several defects during oogenesis. The most obvious phenotypes manifested are the dorsal-ventral patterning defects of the egg and the organization of the oocyte nucleus karyosome. Recent studies have offered some insight into the connection between activation of the meiotic checkpoint and the karyosome. It was found that nucleosomal histone kinase-1 is essential for karyosome formation. NHK-1 phosphorylates the linker, BAF, to release meiotic chromosomes from the oocyte nuclear envelope during karyosome formation. Expression of a non-phosphorylatable BAF3A mutant prevented.