PAK4 and cancer: P21-activated kinase 4 (PAK4), a representative member of class II in the PAK family of serine/threonine kinases, is originally identified as a cytoskeleton regulatory protein[2] and plays key roles in cell survival, anchor-independent growth, and the formation of filopodia [2–7], especially in tumorigenesis and development of gastric cancer [8–10] and other malignant tumors [6,11–18].