*2.3. Flow Cytometry and Aldehyde Dehydrogenase (ALDH) Assay*

Irradiated cells were gently dissociated and incubated with anti-CD44-Fluorescein (FITC) or anti-CD133-phycoerythrin (PE) antibody (eBioscience Inc., San Diego, CA, USA) for 30 min on ice. The samples were washed twice with PBS containing 0.1% BSA and EDTA, and the cells were resuspended in PBS containing 0.1% BSA and EDTA and analyzed using FACSCalibur and CellQuest programs (BD Biosciences, San Jose, CA, USA). For ALDH assay, an ALDELUOR kit (Stemcell Technologies Inc., Vancouver, BC, Canada) was used to detect the cell population with high ALDH enzymatic activity, according to the manufacturer's procedure, after which the cells were analyzed by flow cytometry. Apoptotic cells were analyzed as previously described [17].
