A postage-stamp-sized platform may be offering scientists one of the most realistic new ways to study human heart valve disease. Heart valves are highly specialized structures that open and close thousands of times each day, directing blood through the chambers of the heart and preventing it from flowing backward. Heart valves in animals can develop at different speeds and may respond differently to genetic mutations, physical stress or metabolic injury. “To study human valve diseases, we need human valve models.” The resulting heart assembloid incorporated two organoid components made from different types of heart cells.