Consistent with the previous results, the present study shows that the number and size of CSC spheres, the expression levels of stem cell markers (c-Myc, Klf4, Oct4, and Nanog), and the percentage of cells with a CD44+/CD24−/low phenotype were significantly increased by hypoxia in both mouse and human breast cancer cells (Figure 1A–1C). Here, KLF4 is linked to breast cancer.