A) The FaDu cell line has been transduced using two different lentiviral constructs, containing single and double cassettes of let-7d. The let-7d expression levels were similar, but the MEAN of GFP fluorescence marker was nearly half lower in the 7d7d line than in 7d. The irradiation effect in both cases was similar. B) Five cell line models based on the FaDu cell line have been stable for further experiments. The expression for each model was calculated, comparing it to the FaDu-GFP cell line, using the 2-ΔΔCt method. The following results were found: A: 8.72; B: 17.44; C22.85; D:37.93, and E:57.89; C) the proliferation ratio generally decreased in the overexpressed let-7d cell line, compared to the controls for the FaDu-GFP line; however, differences between models were not statistically significant, and as such, we did not observe a trend.