APP and Alzheimer disease: Overall, the notion that CSF levels of APP‐derived peptides (including Aβ1‐42) may be also reduced in diseases other than AD argues against the role of Aβ1‐42 levels in CSF as a measure of cerebral amyloidosis only and suggests that it may also reflect other pathophysiological processes (i.e., synaptic loss or neuroinflammation).