Recently, magnetic nanoparticles (MNPs) have been successfully employed to fabricate Förster resonance energy transfer (FRET) sensing platforms for detecting MMP-9 activity both in vitro and in vivo because there is inherent spectral overlap interference of fluorescent dyes and MNPs.39 The MNP-based FRET sensing platform enables to quantitatively map MMP-9 activity across the entire tumor because MNPs can efficiently accumulate in tumors through the enhanced permeability and retention (EPR) effect. The gene discussed is MMP9; the disease is neoplasm.