DNA contains genetic information that acts as a blueprint for how parts of your body are made and work, determining such things as eye and hair color. DNA is made up of long strands of repeating letters that form a code. The order in which these letters are written is very precise. In the same way that a spelling mistake in a word could change its meaning entirely (for example, mean and meat), a single change in the sequence of DNA can lead to a change in how the body works, which may lead to disease. Currently, researchers and doctors know some of the genetic changes that may result in disease, but they do not know all of the genetic changes that can cause disease.