Second, since CD24+/CD44+ and CD24-/CD44+ HNSCC cells were sorted by FACS technology, we cannot rule out the possibility of undetectable residual CD24+/CD44+ cells contaminating the CD24-/CD44+ cell population, which resulted in CD24+/CD44+ cells within the xenograft tumors, although this was considered a remote possibility. This evidence concerns the gene CD44 and head and neck squamous cell carcinoma.