Researchers have found that two overlapping genes responsible for bipolar disorder. In previous study researchers found the genes increases risk for schizophrenia. This leads to the idea that the genes may be responsible for both disorders. Bipolar disorder, also known as manic-depressive illness, is a brain disorder that causes shifts in a person's mood, with spurts of high energy and elation alternating with longer periods of fatigue and deep sadness. It affects about one percent of adults, usually beginning in late adolescence. In a series of experiments the researchers found strong evidence that one version of a stretch of DNA that includes the two genes was associated with bipolar disorder. This consistency adds strength to the finding. These two reports are the first consistent demonstration of a gene complex associated with both bipolar illness and schizophrenia. For a century, these disorders were thought to be inherited separately. In the past few years, however, the possibility that the same genes contribute to both disorders has gained favor.