Small interfering RNA (siRNA) was conjugated with poly(ethylene glycol) (PEG) at four different terminal ends (sense 3', sense 5', antisense 3', and antisense 5') via cleavable disulfide and noncleavable thioether linkages to evaluate their gene silencing efficiencies upon complexation with Lipofectamine (TM) 2000. The PEGylation site at the four siRNA termini and PEG molecular weight were not critical factors to significantly affect gene silencing activities. Cleavable siRNA-PEG conjugates showed comparable gene silencing activities to naked siRNA, and exhibited sequence-specific degradation of a target mRNA. Interestingly, noncleavable siRNA-PEG conjugates were processed by Dicer, enabling to exert RNAi effect without showing a target sequence-specific manner. However, only cleavable siRNA-PEG conjugates significantly reduced the extent of INF-alpha release as compared to noncleavable siRNA-PEG conjugates, suggesting that they can be potentially used for therapeutic siRNA applications. (C) 2010 Elsevier B.V. All rights reserved.