Research into neurodegenerative disorders such as Parkinson's disease, Alzheimer's disease, Huntington's disease, and amyotrophic lateral sclerosis (ALS) indicate that the cytoskeleton is affected in these diseases. Parkinson's disease is marked by the degradation of neurons, resulting in tremors, rigidity, and other non-motor symptoms. Research has shown that microtubule assembly and stability in the cytoskeleton is compromised causing the neurons to degrade over time. In Alzheimer's disease, tau proteins which stabilize microtubules malfunction in the progression of the illness causing pathology of the cytoskeleton. Excess glutamine in the Huntington protein involved with linking vesicles onto the cytoskeleton is also proposed to be a factor in the development of Huntington's Disease. Amyotrophic Lateral Sclerosis results in a loss of movement caused by the degradation of motor neurons, and also involves defects of the cytoskeleton.