Many of the G2-enriched proteins, e.g., myeloperoxidase (MPO), lactotransferrin (LTF), neutrophil gelatinase-associated lipocalin (LCN2), and matrix metalloproteinase-9 (MMP9) are specific to neutrophils, a subset of white blood cells that are part of the first line of response to bacterial infection. Here, MMP9 is linked to bacterial infectious disease.