Essential subunit of the gamma-secretase complex, an endoprotease complex that catalyzes the intramembrane cleavage of integral membrane proteins such as Notch receptors and APP (amyloid-beta precursor protein). It probably represents a stabilizing cofactor required for the assembly of the gamma-secretase complex (By similarity). Homodimer. Component of the gamma-secretase complex, a complex composed of a presenilin homodimer, nicastrin, aph1 and pen2 (By similarity). Belongs to the nicastrin family.