Variations Causing Harmful Changes
There are a group of variations in coding and regulatory regions that result in harmful effects. These are called mutations. They cause disease because changes in the genome's instructions alter the functions of important proteins that are needed for health. For example, diabetes, cancer, heart disease, Huntington's disease, and hemophilia all result from variations that cause harmful effects.
In a "simple" disease such as hemophilia, variation in one gene is sufficient to cause disease symptoms. By contrast, in a "complex" disease like cancer, symptoms are seen only after many variations have occurred in different genes in the same cell.