Why Does the Same Mutation Kill One Person but Not Another?

For a very long time researchers have studies the role of genetic variability and the role it has in diseases. Genetic and environmental differences are not enough to tell. The majority of genetic disorders have different effects on individuals.  A person who a specific mutation could develop a disease however, someone with the same mutation may not.  This is true for even identical twins. Researchers have found that they needed to find a way to quantify tiny variations in genes and to find the small differences in gene expression in vivo to find out why this happens.