ABCB1 and neoplasm: Conjugating NPs with NO-donors is a novel strategy in antitumor therapy because of its multifunctional effects, including the induction of apoptosis and inhibition of metastasis, reversing the MDR effect via the inhibition of P-glycoprotein expression and improving drug delivery via the enhanced permeability and retention (EPR) effect by stimulating tumor angiogenesis.