The invention is based on the finding that Clp1 has the activity of an RNA kinase. Clp1 is therefore useful as an enhancer of siRNA activity. It can be used per se, in kits or expressed in cell lines. Clp1 transgenic and Clp1 knock-out non-human animals are useful for studying the function of siRNAs. Clp1 is also useful in gene therapy to enhance the efficacy of therapeutic siRNAs.