Our DNA is very long and complicated. It is like a road that twists and turns all the way across the country, and scientists are like drivers on that road trying to find one town, or one street, or even one small house. Road signs and maps help us find our way, and luckily for researchers, DNA has some road signs that they can use to make maps, too. DNA has many markers along it, little pieces that act like road signs. When researchers can find these signs, they can tell where they are, and they know some other things that are around that spot.

Scientists who are trying to find which medicines are best for any one person are making use of these markers. When a medicine works well for a group of people, researchers will learn what the DNA markers look like for these people. Then when another person needs that medicine, doctors will compare the person's pattern with the group's pattern. If the patterns are a good match, then it's a good bet that the medicine will work well for the person, too. If the patterns don't match, then doctors will look for another medicine.

Scientists and doctors can use genes and markers to help diagnose some diseases, too. We know about many diseases that are caused by changes in one gene. Sickle cell anemia, hemophilia and cystic fibrosis are examples of this kind of monogenic (one gene) disease. These diseases usually run in families in predictable ways.