Enter the cells and block protein degradation pathway have been identified. Among them, the proteasome inhibitor MG132 is the most widely used commercial inhibitor for regulating the UPP. Because ZFN levels are directly proportional to ZFN activity, we wished to check ZFN proteolysis with MG132 and determine the effects on ZFNmediated gene disruption. Here, for the first time, we investigated ZFN protein stability. We found that ZFNs undergo proteasomal degradation and that MG132 increases ZFN levels, leading to enhanced genetic modifications by the ZFNs. Our protein stability study should lay the foundation for the advancement of ZFN technology; furthermore, the identification of MG132 as a small molecule that increases ZFN function is expected to facilitate the use of ZFNs. Recently we reported a method to enrich cells with nucleaseinduced mutations by transiently transfecting episomal reporters that encode fluorescent proteins and sorting the cells by flow cytometry. The reporter consists of the mRFP gene, which is constitutively expressed, and the programmable nuclease target sequence followed by an out-of-frame eGFP gene in tandem fashion. Once a double-strand break is introduced into the target sequence by the ZFN, the eGFP gene comes into frame with mRFP because of mutations introduced by a DNA repair mechanism. The expression level of eGFP, determined by flow cytometry, represents the relative nuclease Sodium Danshensu activity of the ZFN, as previously described. We used this system to evaluate the effect of MG132 on ZFN activity. We transfected plasmids encoding ZFN-224 pairs, which target the CCR5 gene, along with a reporter plasmid containing this nuclease��s target site into 293T cells. At 12 hrs post transfection, the cells were split into three 35 mm dishes. The next day, the media was replaced with fresh media containing increasing concentrations of MG132. After 3 days of incubation at 37uC, the cells were prepared for flow cytometric analysis. The results showed that eGFP + cells/mRFP + cells were significantly increased on an average of 1.8-fold in the MG132 treated cells as compared to the untreated cells. Our result indicates that MG132 treatment may enhance ZFN activity, as assayed by the ZFNinduced mutation rate in cells. To NSC 601980 validate the effec
