Probable subunit of the gamma-secretase complex, an endoprotease complex that catalyzes the intramembrane cleavage of integral proteins such as Notch receptors and APP (amyloid-beta precursor protein). It probably represents a stabilizing cofactor for the presenilin homodimer that promotes the formation of a stable complex. Probably present in a minority of gamma-secretase complexes compared to APH1A. Probable component of the gamma-secretase complex, a complex composed of a presenilin homodimer (PSEN1 or PSEN2), nicastrin (NCSTN), APH1 (APH1A or APH1B) and PEN2. Such minimal complex is sufficient for secretase activity, although other components may exist (By similarity). Interacts with PSEN1 and PSEN2. Weakly or not expressed in leukocytes, lung, placenta, small intestine, liver, kidney, spleen thymus, colon, skeletal muscle, heart and brain. Expressed at low levels in most tissues. Belongs to the APH-1 family.