This ratiometric PA system makes it possible to quantitative detect the MMP‐2 expression in the microenvironment of 4T1 solid tumours with different sizes (Figure 5).[39] Through using the similar stratagem, they also developed another ratiometric PA probe, which is composed of a hemicyanine fluorophore, a caspase‐3 substrate peptide sequence DEVD, and a tumour‐targeted cyclic RGD peptide sequence. The gene discussed is MMP2; the disease is neoplasm.