Main genetic factors which may increase the risk of cancer are genetic disorders (mutations), genetic differences which affect the structures and expression levels (carcinogenic metabolism) of enzymes that activate and detoxificate chemicals and polymorphic/genetic changes which affect the capacity to repair DNA damage. Polymorphisms are observed more frequently than the mutations. A gene polymorphism is defined as the occurrence of genetic variants or gene alternative forms in frequencies higher than 1 percent. Single nucleotide polymorphisms (SNPs) are the most observed genetic variants in human genome. Presence of thousands of polymorphic genes in the genome and the fact that the genome may affect the susceptibility to cancer of individuals with these variants, lead many researchers to explore this uncharted study area.