Breast cancer is a complex disease associated with specific morphological and clinical features. Significant health disparity and high mortality rate is reported in African American (AA) patients, who suffer from unique and highly aggressive breast tumors. AA triple negative (TN) breast cancer patients suffer worse outcomes to chemotherapy compared with Caucasian women. Aggressive TN breast tumors contain poorly differentiated cells and express embryonic stem cell specific gene sets. These poorly differentiated mammary cancer stem cells (MCSCs) are the most tumorigenic cell types that drive initiation and progression of breast cancers. RAS/Raf/ERK1/2 signaling cascade has been found to promote every aspect of breast tumor progression including aggressive behavior like high angiogenesis and motility. We hypothesize that Mammary cancer stem cells drive aggressive TN breast tumors in AA women through sustained ERK1/2 signaling. We will test this hypothesis with the following Specific Aims: 1) Establish whether mammary cancer stem cells from AA TN breast tumors form aggressive xenografts in nude mice. 2) Examine whether sustained ERK1/2 increases aggressive behavior of mammary cancer stem cells enriched mammospheres from AA TN breast tumors. 3) Examine whether inhibition of ERK1/2 signaling is effective in attenuating aggressive properties of mammary cancer stem cells from AA TN breast tumors. Fluorescence-activated cell sorting (FACS) will be used to enrich MCSCs (Lin-/CD44+/CD24- /ALDH1+) from TN AA and Caucasian breast tumors, which will injected into the nude mice to form xenotransplants. These xenografts will be used to compare tumor volume, expression of angiogenesis stimulating factors (VEGF, CD31, MMP9), and motility of cells (Boyden Chamber Assay) between the two groups. MCSCs will also be enriched as mammospheres and pERK1/2-mediated effect on cell motility and expression of angiogenesis stimulating factors will be analyzed. ERK1/2 signaling will be attenuated in MCSCs by Nup153 shRNA and its effect on tumor promoting behavior will be analyzed both in vitro and in vivo. Increased understanding of the critical role of ERK1/2 signaling in MCSCs from AA patients and may provide novel targets for therapeutic drug design.