CD44 and head and neck squamous cell carcinoma: To test for differences in the radio-responsiveness of the CD44+/ALDH + phenotype of HPV positive versus negative HNSCC cells, cell cultures were irradiated with a 4 Gy dose and analysed for changes in the proportion of this phenotype over intervals of 24, 48 and 72 hours, reflecting time intervals between fractionated doses in external radiotherapy.