These are human stem cell-derived motor neurons growing in a dish. The red marks the nucleus of each neuron, and the green marks the cell body and long processes, termed neurites. Motor neurons, which control the body’s muscles, normally reside in the brain and spinal cord, making them difficult to access and study. These motor neurons were artificially reprogrammed from stem cells obtained from healthy humans as well as from patients with amyotrophic lateral sclerosis (ALS), a neurodegenerative disease affecting motor neurons. Once motor neurons die, they cannot be replaced, and ALS patients gradually lose muscle control and die within a few years. By directly observing morphological and functional changes as ALS patient-derived motor neurons age, we can better understand the earliest events causing motor neuron dysfunction in ALS.