PSEN1 is a protein involved in the gamma-secretase complex, which is crucial for the processing of several proteins, including the amyloid precursor protein (APP) and the Notch receptor. Mutations in the PSEN1 gene are associated with early-onset familial Alzheimer's disease (AD) and can lead to an increased production of amyloid-beta, the main component of amyloid plaques found in AD brains. Over 300 PSEN1 mutations have been discovered, some of which have been found in atypical AD or non-AD phenotypes such as frontotemporal dementia (FTD), Parkinson's disease (PD), dementia with Lewy bodies (DLB), or spastic paraparesis (SP).