RAC1 and breast neoplasm: As there are no clinically available inhibitors for small GTP binding proteins, we develop an endosomal pH-responsive nanoparticle system to systematically deliver Rac1 siRNA together with cisplatin to breast tumors in vivo and indicating that Rac1 silencing effectively recovers the sensitivity to cisplatin in patient-derived xenografts (PDXs) derived from TNBC patients whose are resistant to platinum-based NAC.