The MDA-MB-231 breast cancer cells, like many human breast cancer cell lines, have high Pak4 levels.9 In this study, we used siRNA to reduce Pak4 levels in MDA-MB-231 cells and found that reduction of Pak4 expression led to dramatic changes in cell growth and morphology that were consistent with at least partially reversing the malignant transformation process. The gene discussed is PAK4; the disease is breast cancer.