Researchers have developed a way to grow human heart tissue that can serve as a model for the upper chambers of the heart, known as the atria. The tissue, derived from human induced pluripotent stem cells (hiPCSs), beats, expresses genes, and responds to drugs in a manner similar to a real human atrium. The model may be useful for evaluating disease mechanisms and drugs for atrial fibrillation -- the most common type of arrhythmia.