To evaluate the practicality of the proposed method for detection of miR-21, RNA from the two human cell lines MCF7 (breast cancer) and MCF 10A (immortalized normal breast cells) was used in the subsequent in vitro experiments. As a negative control, we used RNA from both cell lines that had been extracted with the RNeasy kit which removed virtually all small RNAs. The RNeasy-treated samples generated a very weak fluorescence signal (Fig. ). Therefore, the remaining larger RNA molecules were unable to recover the fluorescence of FAM-ssDNA. In contrast, total RNA from MCF7 and MCF 10A generate obvious fluorescence signal. The fluorescence signal for the MCF7 cancer cell line was higher than for the normal MCF 10A cells, consistent with the higher expression of miR-21 in MCF7 vs MCF 10A. The result indicated that the FAM-ssDNA/DFNS-4@DA complex can differentiate between cancer and normal cells, which was similar to the result of the literatures .