Neurofibromatosis type 1, or von Recklinghausen disease, is an autosomal dominant disorder with high penetrance, but variable expressivity. The disease has three major features: (1) multiple neural tumors anywhere on or in the body (2) numerous pigmented cutaneous lesions (cafe au lait spots); and (3) pigmented iris hamaomas (Lisch nodules). Electron micrographic studies show that the tumors are the result of the proliferation of fibroblasts or Schwann cells in the peripheral nerves, possibly due to ras inactivation. There is no treatment, except for surgical resection of symptomatic tumors. Ependymoma can occur wherever ependymal cells are found. They are more common in children, and most often originate in the fouh ventricle. They are the most common intramedullary glioma of the spinal cord. Huntington disease, an autosomal dominant disorder, is characterized by severe degeneration of the caudate nucleus along with degenerative changes in the putamen and coex. In addition to chorea, these patients frequently suffer from athetoid movements, progressive dementia, and behavioral disorders. Marfan syndrome is due to a defect in the gene for fibrillin. The major clinical findings involve the skeleton, cardiovascular system, and the eye. Affected individuals tend to be tall with long extremities and long, tapering appendages. Mitral valve prolapse and dilatation of the aoic valve ring or aoic dissection due to cystic medial degeneration are common.