Macrophage migration is facilitated toward hypoxic regions [e.g., via hypoxia-inducible factor-1α (HIF1α) as a chemoattractant; ref. 17] but can be inhibited by dense ECM (primarily comprised of collagen; ref. 18), promoted by fibrogenic activity [identified by alpha smooth muscle actin (αSMA); ref. 19] and breached by tumor cells via matrix metalloproteinase 9 (MMP9; ref. 20). The gene discussed is MMP9; the disease is neoplasm.