Finally, there are genetic variations that have "latent" effects. These variations, found in coding and regulatory regions, are not harmful on their own, and the change in each gene only becomes apparent under certain conditions. Such changes may eventually cause some people to be at higher risk for cancer, but only after exposure to certain environmental agents. They may also explain why one person responds to a drug treatment while another does not.

Here is part of the genome from two people who are both smokers and drinkers, but only one of them gets cancer. The zoom into the chromosomes of these two men shows just a sampling of the differences in variation that are responsible for their individual cancer risk. The variations themselves do not cause cancer. They only affect each person's susceptibility to tobacco smoke and alcohol after exposure.