The present invention provides 4-nucleotide (4mer) RNA motifs that confer immunostimulatory activity, in particular, IFN-α-inducing activity to a RNA oligonucleotide. The present invention also provides RNA oligonucleotides, including siRNA, with high or low immunostimulatory activity. The present invention further provides the use of the RNA oligonucleotides of the invention for therapeutic purposes.