Studies, including our own, have shown that neutrophils can promote tumor cell proliferation, inhibit apoptosis, enhance angiogenesis, create an immunosuppressive microenvironment, and facilitate premetastatic niche formation by secreting myeloperoxidase (MPO), arginase 1 (ARG1), and neutrophil extracellular traps (NETs) [19–22]. Here, MPO is linked to neoplasm.