Mechanism of Janus role of the (MTX + PEG)-CS-NPs. Once intravenously administrated, it was anticipated that the (MTX + PEG)-CS-NPs were accumulated at the tumor site by the EPR effect. Prior to the cellular take, the (MTX + PEG)-CS-NPs were served similarly as a targeted drug delivery system, in which MTX can function as a targeting moiety and selectively transport the NPs to the target cells. Once internalized into the target cells, the (MTX + PEG)-CS-NPs were served similarly as a prodrug system, in which MTX would be released inside the cells and function as a therapeutic anticancer agent. Additionally, the protease-mediated drug release could ensure that MTX timely change its role from targeting (via FA receptor-mediated endocytosis) to anticancer (inhibit DHFR activity and stop FA cycle). This work systematically revealed the unanticipated targeting coordinated with anticancer efficiency of Janus-like MTX in vitro.