  Alzheimer's disease causes severe neurodegeneration in the brain that leads
to a certain death. The defining factor is the formation of extracellular
senile amyloid plaques in the brain. However, therapeutic approaches to remove
them have not been effective in humans, and so our understanding of the cause
of Alzheimer's disease remains incomplete. Here we investigate physical
processes that might relate to its onset. Instead of the extracellular amyloid,
we scrutinize the intracellular domain of its precursor protein. We argue for a
phenomenon that has never before been discussed in the context of polymer
physics: Like ice and water together, the intracellular domain of the amyloid
precursor protein forms a state of phase coexistence with another protein. This
leads to an inherent instability that could well be among the missing pieces in
the puzzle of Alzheimer's disease.
