In this study, we aimed to identify its target gene and its critical role in CRC. In this study, a clear connection between miR-18a and the gene ATM was established. Luciferase reporter assay and western blot analysis confirmed the interaction, which was through the binding of miR-18a to the 39 untranslated region of ATM mRNA and subsequently suppressed its protein translation and its activity. This association was most evident from the inverse correlation between miR-18a and ATM in rectum tumor tissues. ATM is a high molecular weight protein kinase that plays a central and early role in promoting repair of DNA DSBs, which are one form of the most cytotoxic DNA lesions that arise through both endogenous and exogenous sources. In unstimulated cell, ATM mostly exists as an inactive homodimer or AbMole Metaproterenol Sulfate multimer, with the kinase domain of one ATM protein bound to the internal domain of another ATM protein containing the serine 1981phosphorylation site. This structure is essential to keep ATM protein inactive and stable when there is no DNA damage. Therefore, under no external stimulus that leads to DNA damage, miR-18a over-expression induced no significant phenotypic change in HT-29 and HCT-116 cells as evident by cell viability, proliferation and apoptosis analysis compared with control groups. However, in response to DNA damage induced by etoposide, a genotoxic agent that specifically induces DSBs, we found that cells over-expressing miR-18a were less able to restore from damage compared to cells transfected with miRNA control precursor, reflecting a compromised DNA DSBs repair mechanism. Under DNA damage stimulus, the kinase domain of one ATM protein phosphorylated the 1981-domain of the interacting ATM protein, resulting in active kinase in monomeric form. Activated ATM is freed and can phosphorylate a diverse array of downstream targets that participate in events to repair the DNA damage. Over-expression of miR-18a reduced ATM protein amounts and thus the availability of activated ATM for DNA repair. Therefore, as DNA damage accumulates without prompt repair, miR-18a over-expressing cells were more prone to commit apoptosis, reduced clonogenic survival and proliferation rate, as evident in both HT-29 and HCT-116 cells. Compromised DNA repair mechanism due to loss of ATM function is a known predisposition to various diseases. The most evident example being the inherited autosomal recessive disorder, Ataxia-telangiectasia, which results from loss of ATM protein expression or functional protein product. The disease is characterized by progressive cerebellar ataxia, neuro-degeneration, radiosensitivity, cell-cycle checkpoint defects, genome instability, and a predisposition to various forms of cancer. Chromosomal gain in region 13q31.1, where miR17-92 is located, is an early event in the adenoma-carcinoma sequence. Consistently, up-regulation of miR-18a is found since precancerous stage of CRC. The suppressed DNA repair mechanism induced by up-regulated miR-18a could possibly serve a catalyzing role in the formation of carcinoma. Currently, tumor stage is the most important prognostic indicator for CRC patients. Nevertheless, many patients developed recurrence after surgical resection regardless of stage or the provision of adjuvant chemotherapy. Additional prognostic biomarkers are needed to provide better recurrence risk assessment so patients can benefit from close follow-up.