APP and Alzheimer disease: Whereas in conventional AD the AβPP-independent C99/iAβ production pathway is always self-sustainable from the instance of its activation, in unconventional AD it takes some time for it to become self-sustainable: if the initial ISR-eliciting unconventional stressor were withdrawn too early, the activity of the AβPP-independent C99/iAβ generation pathway would be only transient.