 Cells are microscopic in size, but some, amazingly, can be over four feet long. These are the motor neurons, the longest cells of the body. As their name implies, motor neurons are in charge of movement. The long motor neurons extend from the brain, all the way down through the spinal cord, and then through another set of motor neurons to muscles throughout the body. When we want to reach for a cup of tea, our brains activate a very complex, specific combination of motor neurons that transmit a signal. The signal travels down the motor neuron from the brain through the spinal cord, through another motor neuron down the arm, and triggers the muscle contraction, which allows our hands to reach for the tea. Even simple movements like these require enormous processing power and coordination in the brain. If motor neurons are not functioning properly, our foot might move instead of our hands, or we may not be able to move at all. Loss of motor neurons causes paralysis, resulting in difficulty speaking, swallowing, and eventually breathing, leading to respiratory failure and ultimately death. This is the cause of a rare, devastating disease called ALS, for amyotrophic lateral sclerosis, which has an average survival of two to five years. ALS gained recognition in the United States when baseball legend Lou Gehrig developed the disease and died of ALS two years later. There is currently no cure for ALS and limited treatments to slow or mitigate the symptoms. With funding from the California Institute for Regenerative Medicine, CIRM, California's stem cell scientists are working determinately to make an impact. So far, we know that ALS is caused by the degeneration and eventual death of motor neurons throughout the body. In 5 to 10 percent of cases, ALS runs in families and is caused by genetic mutations. In 90 to 95 percent of cases, ALS occurs sporadically. After many years of research, scientists have discovered that degeneration of motor neurons in ALS may be caused by a malfunction in surrounding cells called astrocytes. Astrocytes are present in the brain and spinal cord where they support the motor neurons by providing nutrients and maintaining a healthy environment for them to reside. You can think of the astrocytes as the motor neuron's personal assistant. While the motor neuron is hard at work, the astrocytes feed the motor neuron, remove waste, and provide a healthy environment. In the case of ALS, astrocytes can malfunction, which causes the motor neurons to degenerate and eventually die. Now that we have several strong hypotheses for why motor neurons die in ALS, we can devise two theoretical options for treating ALS. One, replace the dying motor neurons, or two, replace the surrounding cells that provide the optimal environment for motor neurons. Because motor neurons are so long and complex, we can't transplant them into the body. Instead, scientists are using stem cells to find ways to prevent the motor neurons from dying by replacing damaged astrocytes. One group of scientists led by Clive Sensen, PhD, who is director of Cedar Sinai's Board of Governors Regenerative Medicine Institute, is attempting this approach, using astrocyte precursor cells. While astrocytes may provide great support on their own, Sensen has further modified the cells to secrete a powerful growth factor called GDNF, which has been shown in laboratory animals to protect motor neurons. The idea behind Cedar Sinai's approach is to introduce these modified neural precursor stem cells into the spinal cord where they would act as Trojan horses to introduce the protective growth factor GDNF at the site of motor neuron injury. Once implanted, the scientists think these modified cells would migrate to the site of injury where they would turn into astrocytes and produce high amounts of GDNF to protect, support, and maybe even heal the damaged and dying motor neurons. A phase one slash two A clinical trial to test this approach is currently underway at Cedar Sinai. Other groups, including one led by Dr. Goldstein at UC San Diego, are also attempting to introduce neuronal stem cells to replace damaged astrocytes. As scientists and patient advocates build on the progress that Proposition 71 funding has enabled, we must keep the momentum going, understanding that there is still much work to be done. We must remember that human trials will celebrate successes, but barriers will surface along with complications and challenges, so patience and understanding of the scientific discovery process are essential.