Tumours originating from the thymic epithelial cells are called thymomas. They most often occur in adults older than 40. Tumours are generally detected when they cause symptoms, such as a neck mass or affecting nearby structures such as the superior vena cava; detected because of screening in patients with myasthenia gravis, which has a strong association with thymomas and hyperplasia; and detected as an incidental finding on imaging such as chest x-rays. Hyperplasia and tumours originating form the thymus are associated with other autoimmune diseases - such as hypogammaglobulinemia, Graves disease, pure red cell aplasia, pernicious anaemia and dermatomyositis, likely because of defects in negative selection in proliferating T cells.