CD44 and lip and oral cavity carcinoma: Subpopulations of oral cancer cells that highly express CD44 and show enhanced epidermal surface antigen (ESA) [47] or an elevated expression of stage-specific embryonic antigen-4 (SSEA-4) [48] have the characteristics of CSCs and show high tumorigenic potential upon transplantation (CD44high/ESAhigh; 5 × 103 cells, CD44high/SSEA-4high; 1 × 104–2 × 105 cells) into the tongue of immunodeficient mice.