The most common cause of SMA is a mutated or missing gene called 'survival motor neuron 1' (SMN1). This condition is inherited in an 'autosomal recessive' pattern. If both parents are carriers, there is a 25% (one in four) chance that their child will develop SMA. Approximately one in 40 individuals is a carrier of the gene variation that causes SMA. SMA occurs when a child inherits the mutated gene from both carrier parents, often without the parents being aware of their carrier status.