Multidrug resistance(MDR)is a major problem in cancer chemotherapy.Directly delivering drug-loaded vehicles into the nucleus of MDR cancer cells through nucleopores is an efficient strategy to bypass drug-resistant barriers.Most recently we developed a size changeable polymer micelle system with a dual-shell composed of mPEG-PLA-ss-PEI-DMMA polymer for combating drug resistance in human breast cancer cells(1-8).These micelles latently deactivate PEIs amines to negatively charged acid-labile amides to inhibit nonspecific interaction with normal cells but regenerate the original PEI once in acidic tumor tissues and subsequently protonate PEIs amines,resulting in an increase in micellar size.