In order to help treat human diseases, it is important to understand what causes them to occur. Understanding what causes a disease is the first step in understanding the entire abnormal course of disease. Sometimes it is fairly easy to determine what causes a disease. For example, pneumonia is caused by the Pneumococcus bacterium. However, in other cases it is not nearly as easy to tell what is causing a disease, so scientists look for clues from a number of different sources.

One such clue is having a disease run in families, which suggests that a disease might be caused by a gene or genes passed from parent to child, perhaps over many different generations. The process of identifying these genes, called disease gene discovery, is important because it helps scientists to understand what is going wrong as a result...