Lung cancer accounts for 20% of cancer incidence in the United States and 25% of cancer-related deaths. Approximately 80% of lung cancer cases are non-small cell lung cancer (NSCLC). Deficiencies in DNA repair capacity (DRC), apoptosis control and cell cycle checkpoints have been implicated in the pathogenesis of lung cancer including NSCLC.1–3 Therefore, the essential role of the ATM protein in double strand break (DSB) DNA damage response and the importance of the DNA repair system in tobacco-related carcinogenesis highlight the potential significance of ATM sequence variations on NSCLC risk. ATM is a tumor suppressor gene frequently mutated in patients with Ataxia Telangiectasia (AT), a rare form of an autosomal recessive malignancy-prone disorder prominently characterized by extremely high sensitivity to ionizing radiation or other DSB-inducing agents.4 ATM encodes a 370-kDa phosphoinositide 3-kinase (PI3K) protein that belongs to the PI3K-like Serine/Threonine protein kinase (PIKK) family. This family functions in DNA damage responses by phosphorylating proteins in various damage-related pathways.5 ATM exists in an inactive multimer form in the cell nucleus, which dissociates into monomers upon exposure to DSB-inducing genetic insults.6 The interaction between the MRN (MRE11, RAD50, NBS1) complex and ATM in the presence of damaged DNA yields a more than 80-fold increase in ATM kinase activity, which is capable of relating the signals to a plethora of downstream effectors through phosphorylation of specific serine or threonine amino acid residues.7 The converging effects of these protein effectors control the outcome of the damaged cells through the regulation of cell cycle arrest, DNA repair and apoptosis. Aberrations of ATM protein have been implicated in the etiology of many cancers,8–13 including lung cancer. Through phosphorylating p53 and MDM2 proteins, ATM disrupts the p53-MDM2 interaction and thus, increases the nuclear accumulation of p53.5 Consistent with this notion, Bartkova et al. found that ATM, CHK2, p53 and H2AX were highly expressed and phosphorylated in early precursor lesions of various cancers, suggesting that the ATM-CHK2-p53 axis plays an essential role in the DNA damage-response in the early development stage of these malignancies, including lung cancer.14 Moreover, Eymin et al. reported that the ATM/CHK2 pathway also mediates the p14ARF-induced G2 cell cycle checkpoint arrest in response to DNA damaging agents. This pathway is independent of p53 activation and its defects contribute to lung carcinogenesis.14 Taken together, these observations highlight the pivotal role of ATM in the prevention of lung cancer development through the modulation of multiple pathways. However, most previous studies focused on the carcinogenic effect of ATM rare mutations rather than common variants. In a few studies in which ATM polymorphisms were investigated, controversial results have been reported in terms of the involvement of ATM polymorphisms in the etiology of malignancies such as breast and colorectal cancers.15–20 There have not been any studies of ATM polymorphisms and lung cancer risk in Caucasians. In addition, although ATM haplotypes have also been associated with altered cancer risk, most published studies focused on only breast cancer and the haplotypes in these studies were composed of only potential functional SNPs but were not based on haplotype tagging SNPs (htSNP).15,21–23 Since Bonnen et al. reported extensive linkage disequilibrium (LD) across the complete ATM locus which suggested that few htSNPs were required to construct complete high-power haplotypes to capture common ATM polymorphisms,24 a comprehensive approach combining the power of htSNPs and functional SNPs may provide more clues to the assessment of ATM sequence variants on cancer risk. To test the hypothesis that common ATM sequence variants may modulate NSCLC risk, we assessed the associations of 11 potential ATM htSNPs and functional SNPs with NSCLC risk in Caucasians. In addition, we performed a functional assay to evaluate the physiological significance of the observed associations through genotype–phenotype correlation analyses. To the best of our knowledge, this is the first epidemiological study examining the role of ATM polymorphisms in NSCLC risk in Caucasians.