These conflicts may be explained by sub-classification of CSCs according to their expression of marker proteins which may have different positions/roles in cancer as suggested by Geißler et al. Their data revealed that the amounts of CD44 and ALDH1A1 vary; while ALDH1A1 high tumor cells express low levels of CD44 and EGFR, ALDH1A1-/CD44+ high tumor cells express high levels of EGFR in HNSCC. The gene discussed is ALDH1A1; the disease is head and neck squamous cell carcinoma.