CD44 and head and neck squamous cell carcinoma: Sun and Wang (2011) looked at CMET+ cells compared to CMET- and found increased tumorigenicity in a flank injections and found higher percentage of implantation in CMET+ cells compared to CD44+ cells and slightly lower than ALDH+ cells. In addition, Zhang et al. (2009) looked at HNSCC cell lines and oral cavity primary tumors identified the presence of SP cells. In vitro and in vivo analysis demonstrated SP cells had greater clonal expansion and greater tumorigenicity relative to non-SP cells.