Maintaining a functional blood–brain barrier (BBB) is crucial for brain function, but human models are needed to better understand human disease. ISD researchers developed a human BBB model from stem cells that recreates key features of brain vasculature and allows investigating disease mechanisms and therapeutic interventions. When they inactivated the stroke-risk gene FOXF2 in the BBB, the barrier became dysfunctional, which they could alleviate using an LNP-based treatment.