The blood-brain barrier is a double-edged sword that protects our brains from toxins and other threats that may lurk in circulating blood, but also may prevent passage of helpful drugs into the brain and biomolecules required for healthy brain development. To study the blood-brain barrier’s double-edged nature, researchers from the University of Wisconsin-Madison and Cedars-Sinai in Los Angeles have created the first demonstration of using a patient’s cells to model a blood-brain barrier defect. The new research establishes a laboratory model for Allan-Herndon-Dudley syndrome and hints at therapies that could prevent or reduce the debilitating effects of the disease. Allan-Herndon-Dudley syndrome is caused by defects to a single gene that controls the flow of thyroid hormone to the brain. It affects the developing male brain beginning before birth and results in moderate to severe cognitive disability, impaired speech, underdeveloped muscles and involuntary movement, among other symptoms. As patients age, the condition progresses and many become confined to wheelchairs. One researcher said, “The blood-brain barrier forms pretty early in gestation, so the thyroid hormone, even from the mother, is probably not getting through the barrier and into the brain, likely leading to developmental deficits.” Several other neurological diseases like Alzheimer’s disease and Huntington’s disease may involve dysfunction in the blood-brain barrier.