miR-125a-5p modulates sensitivity to ionizing radiation. HN5 (A) and UM-SCC-22B (B) cell lines were transfected with miR-125a-5p mimic, miR-125a-5p inhibitors, or control miRNA. Cells were then harvested and plated. Four hours after plating, cells were then treated increasing doses of ionizing radiation using a Cesium-137 irradiator. After two weeks of growth, colonies, consisting of 50 cells or more, were fixed with a solution of glutaraldehyde and stained with crystal violet. Colonies were then counted and plotted with surviving fraction as a function of radiation dose. Transfection of miR-125a-5p mimics results in increased sensitivity to ionizing radiation, compared to the cells transfected with control miRNA, while transfection of miR-125a-5p antisense inhibitors rendered the cells more resistant to ionizing radiation. Experiment was carried out in triplicate. Data were analyzed with the Student's t-test at each ionizing radiation dose level and are presented as the mean ± standard deviation with * signifying P ≤ .05, ** signifying P ≤ .01, and *** signifying P ≤ .001.