As BACE1 is the initiator of amyloid beta production, it is the prime therapeutic target for AD. Currently, several BACE1 inhibitor drugs are being tested in clinical trial for AD. However, the safety of BACE1 inhibition remains a concern. We found that age-dependent elevation of APP is associated with decreased BACE1 processing of CHL1 in 5XFAD and in human brains from subjects affected by Down Syndrome (DS). Given that BACE1 cleaves preferentially other substrates over APP, the degree of BACE1 inhibition required to reduce amyloid beta levels may be sufficient to severely impair the processing of other BACE1 substrates. More importantly, our findings indicate that APP elevation, such as DS, could further potentiate the effect of BACE1 inhibition on the processing of substrates like CHL1 and increase the possibility of mechanism-based site effects. My project will focus on the effect of elevated APP levels on BACE1 substrates processing using human neuron derived from iPSCs.