Interestingly in AD, the decrease in CSF Aβ42 associated with its aggregation in amyloid plaques is also correlated with other Aβ isoforms, including Aβ40 and soluble APPs, which are released mainly during physiological synaptic plasticity (Cole and Vassar, 2008; Gabelle et al., 2010). Here, CTSB is linked to Alzheimer disease.