Abstract : A 350-kDa protein is encoded by the ataxia telangiectasia mutated gene (ATM), a potential breast cancer risk gene. and is one of the fundamental components of the DNA-damage response system. The current study aimed to determine the function of inhibitory and regulatory gene ATM in the etiology of breast cancer in Iraqi women by identifying radiotherapy's detrimental impact on molecular factors about 10-20 Gy locally gamma cells, which may have an impact on cancer patients' genetic material (DNA). At the Al-Amel National Hospital for Cancer Management in Baghdad, this study was conducted on 18 breast cancer patients from Iraq. over three months eighteen healthy ladies, aged 35 to 45 years, are non-smokers and non-alcoholics. The results of gene expression show a significant difference in the ATM treatment comparison between the initial and final administration of RT. P value= 0.0047. Specialized primer genes were chosen for the mRNA genes ATM, as well as the primers for the internal control (actin) gene. which has a relationship with Ionizing radiation, using RT-PCR to analyze gene expression. According to the findings, radiation activates the ATM gene, which is involved in the cell cycle, DNA repair, and apoptosis regulation.