Spinal muscular atrophy 1 (SMA1), also known as Werdnig Hoffmann disease, is a genetic neuromuscular disorder that affects the nerve cells that control voluntary muscles (motor neurons). Without treatment, symptoms of SMA1 become apparent before 6 months of age and include worsening muscle weakness and poor muscle tone (hypotonia) due to loss of the lower motor neurons in the spinal cord and brain stem. Feeding and breathing problems may also present. SMA1 is caused by changes (pathogenic variants also called genetic changes) in the SMN1 gene and is typically inherited in an autosomal recessive manner. Diagnosis of SMA1 is suspected by symptoms and confirmed by genetic testing. SMA has been added to the list of recommended newborn screening tests in the United States, so that it can be detected prior to symptoms developing.