RNA interference is a potentially useful way to study cancer targets in vivo, but is hampered by delivery issues. Using the ID4 oncogene in ovarian cancer as an example, Ren et al. developed a tumour-penetrating nanocomplex made up of small interfering RNA (siRNA) complexed with a tumour-penetrating and membrane-translocating peptide, which enabled the specific delivery of siRNA deep into the ovarian tumour parenchyma. Treatment of ovarian tumour-bearing mice with the ID4-specific nanocomplex suppressed tumour growth and improved survival.