PAGE 7 OF 8 Nucleic Acids Research, 2005, Vol. 33, No. 22 e190 potential illumination of entire biological pathways. Use of commercial QDs in the amounts speciﬁed would be reasonably economical- approximately the same cost as the liposome reagent and approximately four times lower than the synthesis of dye-labeled RNA. As shown in previous work, QDs are compatible with a variety of transfection techniques (other reagents, electroporation and microinjection) and there- fore amenable to nucleic acid monitoring in cells that are susceptible to liposome-triggered cytotoxicity (44). Primary cells may be particularly suited to beneﬁt from this method, as non-viral delivery of siRNA has to date been technically dif- ﬁcult. Thus, ODs are versatile, photostable probes that offer an added dimension to improve the power of RNAi as an experimental tool. SUPPLEMENTARY DATA Supplementary Data are available at NAR Online. 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