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:12297508, PubMed:12522139, PubMed:12763021, PubMed:12679784, PubMed:25043039, PubMed:26280335, PubMed:30598546, PubMed:30630874). Required for normal gamma-secretase assembly (PubMed:12522139, PubMed:12471034, PubMed:12763021, 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 (PubMed:12297508, PubMed:12740439, PubMed:19369254, PubMed:25043039, PubMed:26623517, PubMed:26280335, PubMed:30598546, PubMed:30630874). Predominantly located in the endoplasmic reticulum and in the cis-Golgi. Widely expressed. Expressed in leukocytes, lung, placenta, small intestine, liver, kidney, spleen thymus, skeletal muscle, heart and brain. Isoform 1 and isoform 2 are nearly expressed at the same level. Belongs to the APH-1 family.