The causes of schizophrenia are unknown, although the disease has a strong genetic component. Studies of identical twins show that if one twin has schizophrenia, there is a 50% chance that the other twin will have it, too. Since identical twins share identical genes, this indicates that schizophrenia has an equally strong environmental component, the nature of which has not been identified. Current treatments for schizophrenia focus on brain pathways that use dopamine as a neurotransmitter. Despite their ability to alleviate symptoms, many of the drugs used to treat schizophrenia have such negative side effects that patients frequently stop taking them. Now that the human genome has been sequenced, there is a vigorous effort under way to find the mutant genes that predispose a person to the disease. This effort includes sequencing DNA from families with a high incidence of schizophrenia.