According to a variety of studies, MMP-9 expresses higher in thyroid tumor tissues than in non-tumor tissues, in malignant tumors than in benign tumors, in metastatic tumors than in non-metastatic tumors, and before operation than after operation in general, mainly detected by immunohistochemistry (IHC), gelatin zymography, western blotting, reverse transcription-polymerase chain reaction (RT-PCR) and enzyme linked immunosorbent assay (ELISA), as shown in Table 3. Here, MMP9 is linked to cancer.