RNA interference provides compelling opportunities to control gene expression in cells and siRNAs therefore represent a family of new drugs with broad potential for the treatment of diverse human diseases. Several recent studies have demonstrated the feasibility of in vivo siRNA delivery, leading to therapeutic effects in mouse models 1 , 2 , 3 , 4 and also in nonhuman-primates 5 , 6 . Nevertheless, efficient in vivo targeted delivery of siRNA into specific cell types, especially those of immune origin, which are important constituents of the tumor microenvironment and active players in promoting tumor progression, remains to be fully explored. One promising approach for targeted delivery of siRNA is the use of aptamers, which are oligonucleotide-based ligands that bind to specific receptors, such as those on tumor cells 2 .