GSH functions as a signaling molecule which activates the breast cancer stem cell phenotype through copper chelation that leads to an inhibition of mitogen-activated protein kinase kinase (MEK) activity, leading to inactivation of MEK1-ERK signaling, FoxO3 dephosphorylation, and nuclear translocation ultimately leading to enrichment of breast cancer stem cells [158]. The gene discussed is MAP2K1; the disease is breast cancer.