Spinal muscular atrophy is an inherited disorder caused by loss of nerve cells in the spinal cord. These nerve cells control the ability to sit up, crawl, and walk. Activities such as feeding, breathing, and swallowing can also be affected. SMA leads to muscle weakness and wasting away, which become worse over time. Intelligence is not affected by SMA. Age of onset varies from infancy for the most severe form of SMA to adult for the milder late onset form. It is not possible to predict which form of SMA a person might have but the infantile form is the most common. SMA is the most common genetic cause of infant death. Children affected with SMA often have severe muscle weakness before 6 months and die due to respiratory failure before two years of age. There is no cure for SMA. Some of the muscle weakness may respond to treatment. SMA is primarily caused by a common deletion pathogenic variant in the SMN1 gene.