Lipocalin-2 actively encourages the metastasis of breast cancer by inducing angiogenesis, the production of vascular endothelial growth factor (VEGF), EMT, and the migration and invasion of cells, all of which occur through various signaling pathways including the PI3K/AKT/NF-κB [44], HIF-1α/Erk, ERα/Slug axis [45], and stabilizing matrix metalloproteinase-9 [46]. This evidence concerns the gene MMP9 and breast cancer.