The copy number of the neuronal apoptosis inhibitory protein (NAIP) gene, which is located on chromosome 5q13.2, has also been linked to the severity of SMA (although alone it is not a sufficient predictor), since concurrent homozygous SMN1 deletion and NAIP deletion are linked with early onset, severe hypotonia, and a worse outcome or course of the disease [3,4,5]. Here, NAIP is linked to proximal spinal muscular atrophy.