MMP9 and osteosarcoma: Since (i) recent evidence has shown that there is significant crosstalk between osteosarcoma and the immune system [14], (ii) TP53−/− triggers WNT-dependent systemic inflammation that stimulates TAMs to perform breast cancer metastasis [37], and (iii) our data here in a TP53−/−-driven cancer that shows MAPK7 regulates MMP9 and is involved in lung metastasis, we strongly suspected that MAPK7/MMP9-driven TAMs were mediators of osteosarcoma metastasis [14, 37].