MMP9 and non-small cell lung carcinoma: Future studies using single-cell RNA-sequencing approaches will be needed to assess whether increased expression of MMP9 in either cancer cells or distinct cell types in the tumor microenvironment may be linked to poor outcomes and to dissect the contribution of activated MMP9 to NSCLC metastatic dissemination and colonization of multiple organ sites including the brain which is a common site for NSCLC metastasis.