Overexpression of MMP-9 is associated with triple-negative, HER2-positive breast tumors and nodal metastases. (A) Histogram showing percentage of breast cancer patients in each molecular subtype category that express low and high level of MMP-9. Both HER2-positive and triple-negative subtypes demonstrate elevated levels of MMP-9 that are significantly different from those observed in normal breast tissue. The number of patients in each group was mentioned over each bar. The overall relationship between MMP-9 scores and molecular subtypes was evaluated using the chi-square test. (B) Luminal A and (C) Luminal B subtypes showing low level of MMP-9 expression. (D) HER2-positive and (E) Triple-negative subtypes displaying strong cytoplasmic labeling in cancer cells and surrounding stromal cells. (F) Metastatic lymph node demonstrating elevated levels of MMP-9 expression in the cytoplasm of metastatic breast cancer cells. The surrounding lymphocytic and stromal cells did not stain with anti-MMP-9 antibody. Magnification 20X (B-E), 5X (F), 40X inset in Figure 5F.