The most common neurodegenerative disease is Alzheimer's disease, which is characterized by proteins being stored up in the brain and forming so-called plaques and neurofibrillary thread nys, "tangles". In both younger (under-65s) and older people (over 65 years of age), Alzheimer's disease is by far the most common cause of cognitive impairment and dementia. Overall, Alzheimer's disease causes about 2/3 of all cases of dementia. Typical of Alzheimer's disease is that the symptoms debut surreptitiously. Early in the process, memory difficulties are common, that is, the person who is sick does not remember what it was to do, what you were just talking about, what it was you were about to pick up. Other common symptoms may include speech or language difficulties, difficulty finding or understanding words, difficulty counting, deterioration in visuospatial ability (difficulty interpreting visual impressions that may manifest as difficulty reading a text/spatial perception), difficulties with executive functions such as planning, making decisions and getting carried on with something. Social skills such as behavioural and personality changes also occur early in the process. The symptoms of familial Alzheimer's disease can begin as early as the age of 30, 40 or 50, but it is very rare. The earlier the symptoms begin, the more likely it is that the disease is genetically conditioned. In some people with early onset Alzheimer's disease, there is a clear heredity from one generation to the next. Familial Alzheimer's disease, caused by rare mutations/gene changes in three specific genes, presenilin 1, presenilin 2 and amyloid precursor protein (APP). However, it is important to remember that familial Alzheimer's disease overall (i.e. when even late-onset AD is included) is nevertheless still extremely rare and overall this form causes less than 1 of all known cases of dementia. The approximately 600 families/families worldwide that carry the mutation are often aware of this. Thus, the majority of cases of Alzheimer's disease are not hereditary but a sum of inheritance, environment and lifestyle factors.