PAK4 and breast cancer: Further, human breast cancer cells harboring stable PAK4 knockdown grew slower when injected subcutaneously onto the back of immunodeficient mice and formed smaller tumors that were highly positive for SA-β-galactosidase activity.1 We could thus show that several epithelial cancers are susceptible to arrest upon PAK4 inhibition, as suggested for glioblastomas10 further supporting the notion of generalized PAK4 addiction in cancer.5