In conclusion, we show that ZFN proteins have a relatively short half-life and that their turn-over is regulated by the UPP. Furthermore, treatment with the proteasome inhibitor MG132 blocked ZFN protein PF-4217903 cost degradation and extended its half-life, resulting in increased ZFN protein levels and enhanced genetic modification. Our protein stability study should lay the foundation for further development of ZFN technology. The identification of small molecules that increase ZFN protein levels will facilitate the application of ZFNs. To measure the level of functional miRNA in a manner that avoids detecting miRNA mimic trapped in non-functional locations, we immunoprecipitated UV cross-linked RISC from control and transfected cells and measured the amount of RISCassociated miR-200a by deep sequencing of the miRNA-sized RNA fraction in the immunoprecipitate. This revealed that the amount of RISC-associated miR-200a in the transfected cells was approximately equal to the level of other abundant miRNAs. This is proportionally much less than the level of miR200a measured by qPCR, indicating most of the transfected miRNA mimic is not bound to Argonaute and consequently is not functional. Similar results were obtained following transfection of a different miRNA, miR-200b. Thus, although qPCR is a valid technique to measure total miRNA amount, this can be very different from the amount of functional miRNA. Given the majority of miRNA mimic detected by qPCR did not represent the active Argonaute-bound population, we determined its sub-cellular localisation by transfecting a fluorescent siRNA and examining the transfected cells by fluorescence microscopy. The majority of the siRNA did not co-localise with Argonaute, which is consistent with earlier reports of transfected siRNA localising in large cytoplasmic aggregates that are distinct from the GW bodies that are known for their role in RNA silencing. Instead the vast majority of miRNA transfected with either HiPerfect,, RNAi-Max or Lipofectamine 2000 localised with or adjacent to lysosomes, matching earlier reports of lipid-based siRNA transfection. Therefore, the high level of transfected miRNA detected by qPCR is largely attributable to their retention within vesicles and subsequent amplification by qPCR following lysis. Hence, the use of qPCR to measure a miRNA after transient transfection can give the false impression that the miRNA is at massively nonphysiological level, whereas the amount of miRNA bound to Argonaute may indeed be appropriately physiological. On the other hand, it is conceivable that an inefficient 
transfection that results in just a small proportion of cells being transfected could appear to produce an adequate level of miRNA, if measured by qPCR. It is more appropriate to use an assay of miRNA function to verify the effectiveness of the transfection. Of additional interest to users of miRNA mimics for transient transfection, we were able to confirm from our sequencing of the Argonaute-bound pool of small RNAs, that while a miRNA mimic with unmodified passenger strand results in abundant incorporation of the passenger strand into RISC, raising the potential for extensive off-target effects, a mimic that is modified to limit the incorporation of the passenger strand into RISC does indeed achieve this. Although the merits of modified mimics have been previously recognised, published evidence for this is NVP-BKM120 limited to date and has been based largely on reporter assays.