Spinal muscular atrophy (SMA) is a rare, progressive neuromuscular condition caused by a genetic mutation in the SMN1 gene on chromosome 5q. This causes a lack of survival motor neuron (SMN) protein, which causes motor neurones to malfunction, deteriorate and eventually die. People with the condition have a range of symptoms, including muscle weakness, and have worsening physical disability, mobility loss and respiratory dysfunction. SMA can be grouped into 5 main types (types 0 to 4), based on the age of onset and the maximum motor function reached. Type 0 SMA, the most severe, affects babies before birth. The babies do not develop any motor skills and often survive for only a few weeks after birth. Babies with type 1 SMA generally develop symptoms before they are 6 months old. They are unable to sit or roll because of severe muscle weakness, which gets worse over time. The muscle weakness also affects swallowing and breathing, and typically results in death within 2 years. In type 2 SMA, the onset of symptoms is between 6 months and 18 months. People with this condition may be able to sit at diagnosis but are likely to lose this ability over time. However, progressive loss of motor function means they have a reduced life expectancy compared with the general population. In type 3 SMA, there are varying degrees of muscle weakness, which appear between 18 months and 10 years. People with this condition can have a normal lifespan, and walk or sit unaided at some point, but many lose mobility over time. Most people with type 2 SMA and a proportion of those with type 3 SMA will develop scoliosis for which surgery will eventually be needed. Type 4 SMA, the least severe, affects adults, who may have only mild motor impairment and a normal lifespan.