After synthesis on the ribosome, APP enters the endoplasmic reticulum (ER) and is transported via the Golgi apparatus to the cell membrane on the basolateral surface of the cell. The presenilins, which are thought to play a role in APP metabolism, reside mainly in the ER. PAT1, the subject of this article, may attach APP to the microtubules — the 'railroad tracks' of the cell — to assist in transporting APP to the membrane. The generation of the toxic beta-amyloid peptides requires that APP on the cell surface enters a cellular recycling program, after which two cleavages occur, one in the extracellular domain and one in the transmembrane domain. However, which enzymes carry out these cleavages, where in the cell they occur, and how the events are regulated, are not known. An alternative pathway to beta-amyloid formation involves another cleavage by an enzyme termed 'alpha-secretase', which is thought to be located near the plasma membrane. If this second pathway is triggered, beta-amyloid can not be formed, so amyloid plaques do not accumulate.