How does bad amyloid relate to Alzheimer's disease?

The bad type, called beta amyloid or a-beta, is formed when APP gets snipped sequentially by two of these scissors (beta secretase and gamma secretase). As beta amyloid levels rise, gooey clumps of protein -- amyloid plaques -- start to accumulate, gumming up nerve-cell communications and eventually leading to cell death.

Researchers have learned that a critical step in the bad pathway of beta amyloid production is the point at which the parent protein, APP, is snipped by molecular scissors and released by the nerve cell as the highly toxic beta amyloid. Growing evidence suggests that a pair of proteins called presenilins (PS1 and PS2) is required for this final step to occur.