Researchers have turned human embryonic stem cells into spinal motor neurons, the long threadlike cells that carry signals from the spinal cord to the limbs and whose death paralyzes victims of spinal cord injury and disease. As an embryo develops in the womb, a changing mix of chemical factors prompts stem cells to morph into different kinds of neurons at specific times. A group led by Su-Chun Zhang at the University of Wisconsin–Madison hit on the combination of retinoic acid and a signaling molecule called sonic hedgehog to mimic the time when stem cells become spinal motor neurons, allowing them to grow the cells at will. The results, published in the February Nature Biotechnology, should enable scientists to look for drugs and develop treatments to counteract spinal cord disease. The late actor Christopher Reeve had lobbied for stem cell research for precisely this purpose.