Non-catalytic 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) (PubMed:15634781, PubMed:19369254). Required for normal gamma-secretase assembly (PubMed:15634781, PubMed:19369254). The gamma-secretase complex plays a role in Notch and Wnt signaling cascades and regulation of downstream processes via its role in processing key regulatory proteins, and by regulating cytosolic CTNNB1 levels (Probable). The functional gamma-secretase complex is composed of at least four polypeptides: a presenilin homodimer (PSEN1 or PSEN2), nicastrin (NCSTN), APH1 (APH1A or APH1B) and PSENEN/PEN2. Predominantly located in the endoplasmic reticulum and in the cis-Golgi. Full embryonic lethality. No live homozygous embryos are present after 11 dpc. Belongs to the APH-1 family.