An interesting andelegant example of a contrast switchable system using peptides wasdescribed by Song et al., who introduced a peptide-modified magneticresonance tuning (MRET) probe, MPD-1, designed to detect bacterialinfections in a mouse myositis model by responding to matrix metallopeptidase2 (MMP-2) (Figure ). MPD-1 is composed of a magneticiron oxide (Fe3O4) nanoparticle (MNP, a T2 CA) conjugated with gadolinium ions (acting as a T1 CA), with key components including a MMP-2-cleavable self-assembledpeptide (P1) and a bacteria-targeting peptide (P). This evidence concerns the gene MMP2 and myositis disease.