Regulatory RNAs more often than not function by means of direct RNA-RNA binding. The specificity of these interactions is a direct consequence of complementary base pairing, allowing the same basic mechanisms to be used with very high specificity in large collections of target and effector RNAs. This mechanism underlies the post-transcriptional gene silencing pathways of microRNAs and siRNAs (reviewed e.g. in [17]), it is crucial for snoRNA-directed RNA editing [18], and it is used in the gRNA directed mRNA editing in kinetoplastids [19]. Furthermore, RNA-RNA interactions determine the specificity of important experimental techniques for changing the gene expression patterns including RNAi [20] and modifier RNAs [21-24].