To determine the specificity and efficiency of CpG-siRNA uptake, freshly prepared mouse splenocytes were incubated with CpG-linked Stat3 siRNA or an unconjugated Stat3 siRNA, in the absence of any transfection reagent(s). Both the CpG-Stat3 siRNA and unconjugated Stat3 siRNA were labeled with fluorescein isothiocyanate (FITC). Fluorescein-positive DCs, macrophages, B cells, granulocytes and T cells were assessed by flow cytometric analysis, which indicated that the chimeric CpG-Stat3 siRNA was efficiently taken up by both plasmacytoid (CD11c + B220 + ) and conventional (CD11c + B220 − ) splenic DCs, macrophages (F4/80 + Gr1 − ) and B cells (B220 + CD11c − ), but only minimally by splenic granulocytes (Gr1 + F4/80 − ) or T cells (CD3 + ) (Fig. 2a, Supplementary Fig. 2 and Supplementary Table 1 ). This uptake pattern reflects the known distribution of TLR9 expression in murine leukocyte subsets 35 , 26 . Intracellular staining of TLR9 in fixed splenocytes by flow cytometry CD11c + DCs confirmed TLR9 expression (Fig. 2a, bottom right) . Unconjugated Stat3 siRNA was not efficiently incorporated into DCs even after 24 h incubation, demonstrating that linkage to the TLR9 ligand facilitates siRNA uptake (Fig. 2a, bottom left) .