While hereditary disease genes have a high lifelong cumulative incidence rate (penetrance), the penetrance for polymorphism genotypes is not high. Polymorphisms relating to cancer incidence are classified into 1. carcinogen metabolizing enzymes (CYPs, GSTs, NQO1, etc.), 2. DNA repair enzymes (OGG1, XRCC1, XPD, etc.), 3. DNA synthesis and methylation (MTHFR, MS, etc.), 4. cytokines and inflammation-related enzymes (IL-1B, TNF-A, MPO, etc.), and 5. sex hormone metabolizing enzymes and the receptors (CYP19, SRD5A2, ER, etc.). Since genotypes cannot be manipulated, they are not the factors subject to prevention. However, the finding that the strength of association between lifestyle and disease occurrence is influenced by genotypes (gene-environment interaction), opens the door to genotype applications for disease prevention practice.