CD44 and head and neck squamous cell carcinoma: Using both NOD/SCID mice and Rag2/cytokine receptor common γ-chain double knockout (Rag2γDKO) mice, Prince et al., the same group that identified breast CSCs, reported that as few as 5 × 103 CD44+ HNSCC cells could generate tumors in the mice and demonstrated tumor heterogeneity [57].