Perturbations during the DNA Damage Response (DDR) can originate from alteration in the functionality of the microRNA-mediated gene regulation, being microRNAs (miRNAs) small noncoding RNA that act as post-transcriptional regulators of gene expression. By integrating the transcriptome and microRNome, we provided evidence that modeled microgravity can affects the DNA-damage response to IR in human peripheral blood lymphocytes (PBL) (Girardi et al., 2012). By functional assays using luciferase reporter constructs we have shown that miR-27a targets ATM, the gene coding for the main kinase of DDR pathway. Since miR-27a is classified as an oncomir, being overexpressed in several tumors, we investigated the miR-27a-ATM interaction by validating miR-27a as a direct regulator of ATM through site-direct mutagenesis of the reporter vector containing the 3\u2019UTR of ATM gene. We also analyzed the in vitro effects of ionizing radiation in cells overexpressing miR-27a, by assessing cell survival, DNA repair and cell cycle progression. Our results show that overexpression of miR-27a diminishes cellular radiosensitivity, suggesting a role of this miRNA during DDR
