Co‐incubation of 4T1 breast cancer cells with uPA‐PAI‐1‐primed neutrophils, but not direct exposure of uPA‐PAI‐1 protein to the tumor cells or blockade of uPA‐PAI‐1 heteromerization by a novel small‐molecule inhibitor (WX‐340), significantly increased the proliferation of 4T1 cells (Fig 3D; Appendix Fig S2D). Here, SERPINE1 is linked to breast cancer.