A syndrome is a set of medical signs and symptoms that are correlated with each other and, often, with a specific disease. The word derives from the Greek σύνδρομον, meaning "concurrence". In some instances a syndrome is so closely correlated with a pathogenesis or etiology that the words syndrome, disease, and disorder end up being used interchangeably for them. This is especially true of genetically caused syndromes. For example, Down syndrome, Wolf–Hirschhorn syndrome, and Andersen syndrome are disorders with known pathogeneses, so each is more than just a set of signs and symptoms, despite the syndrome nomenclature. In other instances, a syndrome is not specific to only one disease. For example, toxic shock syndrome can be caused by various toxins; premotor syndrome can be caused by various brain lesions; and premenstrual syndrome is not a disease but simply a set of symptoms.