The isolated soft cancer cells (under 400 Pa) but not the stiff cancer cells (over 700 Pa) from CD133−, ALDH1+, or side population-enriched CSCs are capable of initiating tumors when engrafted at a low number of 100 cells into NOD-SCID (severe combined immunodeficient) or immunocompetent mice [63]. This evidence concerns the gene PROM1 and cancer.