Brain organoids are exposed to human serum to mimic the serum exposure consequence of the blood–brain barrier (BBB) breakdown in Alzheimer's disease (AD) patient brains, which recapitulates several AD‐like pathologies, including increased amyloid beta (Aβ) and phosphorylated microtubule‐associated tau protein (p‐Tau) level, synaptic loss, and impaired neural network. The gene discussed is MAPT; the disease is early-onset autosomal dominant Alzheimer disease.