To elucidate whether RAC signaling is also required for BCSC activity, we used two specific RAC-inhibitors with different modes of action in the mammosphere culture of human breast cancer cell lines that are known to generate proliferation-driven mammospheres [28] and represent different breast cancer subtypes: Luminal-A (MCF7 and T47D), Luminal-B (BT474), and HER2 + (JIMT-1). This evidence concerns the gene ERBB2 and breast cancer.