Gal‐9 interacts with Aβ and accelerates its aggregation, generating Gal‐9‐Aβ fibrils, which induce neuroinflammation, synaptic dysfunction and cognitive decline, whereas the knockout of Gal‐9 rescues Aβ deposition, neuroinflammation and learning memory in APP/PS1 mice. The gene discussed is LGALS9; the disease is Mental deterioration.