Essential subunit of the gamma-secretase complex, an endoprotease complex that catalyzes the intramembrane cleavage of integral membrane proteins such as Notch (glp-1 or lin-12). It may represent a stabilizing cofactor for the presenilin homodimer that promotes the formation of a stable complex. Component of the gamma-secretase complex, a complex probably composed of the presenilin homodimer (sel-12, hop-1 or spe-4), nicastrin (aph-2), aph-1 and pen-2. Predominantly located in the endoplasmic reticulum and Golgi region. Expressed from 100-cell stage in most somatic tissues, including neurons, muscle, intestine and developing vulva. Little or not expressed in early embryos. Belongs to the PEN-2 family.