Accumulating evidence supports the concept that cancer stem cells (CSCs) are responsible for tumor initiation and maintenance. They are also considered as an attractive target for advanced cancer therapy. Using a sphere culture method that favors the growth of self-renewal cells, we have isolated sphere-forming cells (SFCs) from cervical cancer cell lines HeLa and SiHa. HeLa-SFCs were resistant to multiple chemotherapeutic drugs and were more tumorigenic, as evidenced by the growth of tumors following injection of immunodeficient mice with 1 x [10.sup.4] cells, compared with 1 x [10.sup.6] parental HeLa cells required to grow tumors of similar size in the same time frame. These cells showed an expression pattern of [CD44.sup.high]/[CD24.sup.low] that resembles the CSC surface biomarker of breast cancer. We further demonstrated that HeLa-SFCs expressed a higher level (6.9-fold) of the human papillomavirus oncogene E6, compared with that of parental HeLa cells. Gene silencing of E6 with a lentiviral-short-hairpin RNA (shRNA) profoundly inhibited HeLa-SFC sphere formation and cell growth. The inhibition of cell growth was even greater than that for sphere formation after E6 silence, suggesting that the loss of self-renewing ability may be more important. We then measured the expression of self-renewal genes, transformation growth factor-beta (TGF-[beta]) and leukemia-inhibitory factor (LIF), in shRNA-transduced HeLa-SFCs and found that expression of all three TGF-[beta] isoforms was significantly downregulated while LIF remained unchanged. Expression of the Ras gene (a downstream component of TGF-[beta]) was also markedly decreased, suggesting that the growth-inhibitory effect could be via the TGF-[beta] pathway. The above data indicate RNA interference-based therapy may offer a new approach for CSC-targeted cancer therapy.