CD44 and breast cancer: To test whether CD44ICD regulates the potential of breast cancer cells for mammosphere formation, we selected MDA-MB-231 and MCF-7 cells and generated stable cell lines, including CD44-depleted cells (CD44KD), CD44-depleted cells with overexpression of CD44ICD (CD44KD/ICDover), and wild-type cells with overexpression of CD44ICD (ICDover), using a lentiviral vector with a genotype that was confirmed by RT-PCR (upper panel) and western blotting (lower panel) (Supplementary Figure S1).