In another approach, Jiang et al. [175] extended the 3' end of the sense strand of a siRNA with a nuclease-resistant DNA hairpin to obtain a so-called "crook" siRNA. This modification had no effect on RNAi-mediated reporter gene inhibition and served as a primer for a filling-in reaction followed by PCR. Parameters were chosen so that the initial rate of template amplification correlates with the initial con-centration of the "crook" siRNA. Under these conditions, quantification of attomolar siRNA levels per cell was possible after liposomal transfections with Oligofectamine.