MMP9 and neoplasm: Quin and co-workers precisely identified the types of M2 type macrophages in heterogeneous tumor microenvironment based on gene expression: PPARG+ macrophages exclusive to normal lung tissue, CCL18+ macrophages cluster and GPNMB, MMP9+ cluster having a role on tumor remodeling, CX3CR1+ cluster involved in pathogen and apoptotic cell clearance (44).