Abstract: Spinal muscular atrophy (SMA) is a genetic condition that results in selective lower motor neuron loss with concomitant muscle weakness and atrophy. The genetic cause of SMA was understood in 1995 when loss or impairment of the survival motor neuron 1 (SMN1) gene was identified as the main contributing factor (Lefebvre et al., 1995). This, in combination with the discovery that humans have a “backup” gene, SMN2, which can produce low levels (approximately 10%) of the full-length functional SMN protein, has led to the generation of SMAspecific gene therapies. SMA was traditionally classified according to age of symptom onset and developmental milestones achieved, with life expectancy and severity varying between individuals. Now, SMN2 copy number is used as a proxy for the prediction of disease severity, with higher SMN2 copy number typically being associated with reduced severity of SMA, although this relationship is not absolute: some individuals with low SMN2 copy number have less severe SMA phenotypes and vice versa. Additionally, the etiology of SMA is further complicated by other factors, such as non-typical nucleotide variants and SMN2-independent modifiers of disease severity.