APP and Alzheimer disease: Thus, the Icelandic Aβ mutation increases by only 30% the efficiency of the intra-iAβ cleavage at the β’-site by BACE1 but this is sufficient to protect from both AD and AACD (aging-associated cognitive decline) because it reduces the rate of accumulation of AβPP-derived iAβ and prevents it from crossing the T1 threshold.