The present invention provides 4-nucleotide (4mer) RNA motifs that confer immunostimulatory activity, in particular, IL-12-inducing activity to a single-stranded RNA oligonucleotide. The present invention also provides single-stranded RNA oligonucleotides, including antisense RNA, with high or low immunostimulatory activity. The present invention further provides the use of the RNA oligonucleotides of the invention for therapeutic purposes.