It is best exemplified by AD occurring in sufferers of a traumatic brain injury not immediately following the event but a decade or two after the event (the lag period is required for the rapid accrual of iAβ produced independently of AβPP followed by the accumulation of AβPP-derived iAβ from the elevated baseline to the T1 threshold; in the absence of the iAβ baseline-elevating event, the T1 crossing would have occurred much later or not at all). Here, APP is linked to Alzheimer disease.