Cancer is perhaps one of the most troublesome diseases, the causes of which are often elusive and unrecognizable. The etiological agents of cancer are difficult to estimate and are not attributed to purely genetic or purely environmental factors, so cancer research has largely focused on the interaction between genes and the environment (Mucci et al., 2001). A large number of studies have investigated the possible causes of various types of cancer, considering diet, heredity and environment as the main determinants. Most diseases are the result of complex interactions between a person's genetic makeup and the environmental agents to which he or she is exposed (NIEHS, 2011). Therefore, the possibility of the involvement of gene-environment interactions in the onset of tumors is being studied worldwide. Gene-environment interactionsA disease occurs when there is an interaction between "genetic susceptibility factors" and "modifiable environmental factors", and variations in the genetic structure of an individual influence his or her susceptibility to the environmental factors responsible for a disease (CDC , 2000). Therefore, due to genetic variation, some individuals are more susceptible than others to a particular disease, and these variations determine the extent of the effect that a particular environmental condition has on an individual's risk of a disease. Gene-environment interactions can be measured Gene-environment interactions can be defined as those that occur when “genetic and environmental exposures work together to cause a pathological outcome in some or all cases” (Brennan, 2002). Because an individual's genetic makeup determines susceptibility to environmental carcinogens, researchers are working hard for the U.S. Department of Health and Human Services' ...... half of document program. Retrieved September 7, 2011, from http://ntp.niehs.nih.gov/ntp/roc/twelfth/roc12.pdfTravis, R.C., Reeves, G.K., Green, J., Bull, D., Tipper, S.J., Baker, K., Beral, V., et al. (2010). Gene-environment interactions in 7610 women with breast cancer: prospective evidence from the Million Women Study. Lance, 375(9732), 2143-2151. Lancet Publishing Group. Retrieved September 7, 2011, from http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(10)60636-8/abstractZhou, W., Liu, G., Miller, D.P., Thurston, S.W., Xu, L.L., Wain, J.C., Lynch, T.J., et al. (2002). Gene-environment interaction for ERCC2 polymorphisms and cumulative exposure to cigarette smoke in lung cancer. Cancer Research, 62(5), 1377-1381. Retrieved September 7, 2011, from http://cancerres.aacrjournals.org/content/62/5/1377.short