1.   However, scientists have never quite understood just how the amyloid precursor protein can lead to the demise of nerve cells.

2.   It is an abnormal variant of a normal brain protein and is made by clipping a long precursor protein at the wrong spot.

3.   The drugs work by blocking an enzyme that carves beta-amyloid, a protein fragement, out of a larger molecule called amyoid precursor protein.

4.   It is different from protease, an enzyme that helps the HIV virus assemble precursor proteins into active proteins.

5.   The enzyme works by cutting beta-amyloid from another compound called amyloid precursor protein, or APP.