MMP9 and cancer: (2) Macrophages produce pro-invasive extracellular matrix-degrading proteases, such as matrix metalloproteinase 9 (MMP9), to promote cancer cell intravasation and metastasis (Pollard, 2004; Noy and Pollard, 2014; Jinushi and Komohara, 2015; Cassetta and Pollard, 2018).