Individuals who had inherited a heterozygous gene mutation, i.e., a mutation inherited only from one parent, may or may not develop a disease resultant from such mutation. The rate of occurrence of a genetic disease among individuals carrying the same gene mutation is known as gene penetrance. Differences in gene penetrance may occur even among individuals with identical genotypes due to several external factors, such as environmental exposure to genotoxic pollutants, inappropriate diet, life style, or drug abuse. The accumulative DNA damage caused by these factors may lead to the damaging of other genes, such as modifier genes, that modulate the mutated gene and its other normal copy, thus contributing to the manifestation of disease, or cause mutation in the other copy (allele) of the gene that was normal at birth. The latter case is known as loss of heterozygosis (LOH), and will induce diseases...