The consequences of IL-19 expression in breast cancer development and progression are as follows: (a) it affects the clinical outcome of breast cancer and tumor metastasis; (b) it increases cell proliferation, cell migration, and fibronectin assembly; (c) hypoxia induces IL-19-mediated gene-expression in cancer cells, which promotes proliferation (IL-1β, IL-6, and TGF-β), migration or metastasis (MMP2, MMP9, CXCR4, and fibronectin), or angiogenesis (MMP2 and MMP9); (d) it facilitates tumor growth and lung metastasis. This evidence concerns the gene MMP9 and breast cancer.