CD44 and head and neck cancer: CSCs have been identified and isolated from various tumors using specific cell surface markers, including CD44 for head and neck cancer [10], CD133 for prostate cancer [11], CD90 for liver tumor [12], CD24 for ovarian cancer [13], CD133 for brain tumor [14], CD44+CD24−/low for breast cancer [15], and CD133 for lung cancer [16].