Presenilin 1 and presenilin 2 are transmembrane proteins belonging to the presenilin family. Mutation of presenilin genes has been linked to early onset of Alzheimer disease, probably due to presenilin's associated ?-secretase activity for amyloid-? protein processing . Endogenous presenilin mainly exists in a heterodimeric complex formed from the endoproteolytically processed amino-terminal (34 kDa) and carboxy-terminal (~20, 22, 23 kDa) fragments (CTF) .