For example, in neurodegenerative diseases, there is a significant proportion (>10%) of monogenic families in whom the disease is caused by singular highly penetrant disease genes, e.g., LRRK2 and PRKN in Parkinson’s disease or PSEN1 and APP in Alzheimer ́s disease [18]. The gene discussed is PSEN1; the disease is early-onset autosomal dominant Alzheimer disease.