Inside our brain cells, a tiny “motor protein” called KIF5A acts like a delivery truck, traveling down long nerve pathways to transport vital cargo like energy-producing mitochondria and waste-clearing lysosomes. In this study, scientists investigated a patient with a rare genetic mutation in this protein—labeled a “variant of unknown significance” because doctors weren’t sure if it actually caused their neurodegenerative disease, Spastic Paraplegia 10 (SPG10). To test it, researchers grew the patient’s nerve cells in a lab using stem cells, and used CRISPR gene-editing to create an identical, healthy version of the cells for comparison. They discovered that the mutation severely damaged the cell’s delivery system: the motor proteins got stuck at the far ends of the nerves, causing traffic jams and structural swelling, which drastically slowed down the travel distance and speed of vital cell cargo. By proving exactly how this mutation breaks down axonal transport, the study officially confirms this genetic variant is a dangerous cause of disease, while showing that stem-cell models are a powerful tool for diagnosing mystery genetic mutations in patients.