Essential subunit of the gamma-secretase complex, an endoprotease complex that catalyzes the intramembrane cleavage of integral proteins such as Notch receptors. It may represent a stabilizing cofactor for the presenilin homodimer that promotes the formation of a stable complex (By similarity). Component of the gamma-secretase complex, a complex composed of a presenilin homodimer (PSEN1 or PSEN2), nicastrin (NCSTN), APH1 and PEN2. Belongs to the APH-1 family.