Understanding the consequences of genetic variation presents another formidable challenge with great societal consequences. In recent years, hundreds of “disease genes” were identified by correlating differences in gene sequence with medical conditions such as colon cancer and sickle cell anemia. Major advances are needed, however, to identify the genes involved in complex “multigenic” traits such as high blood pressure or mental illness. Predicting the response of different patients to drug treatment or other disease therapies based on the genetic makeup holds great promise for pharmacogenomics. Advances in statistics and computer science are needed to distinguish irrelevant genetic differences from those that correlate with changes in function or health.