In the study, which included 30 patients with type 1and 35 with type 2 SMA, deletion of the NAIP gene was found to be associated with the severity of SMA, in particular homozygous deletion of exons 7 and 8 of SMN2 and the absence of NAIP exon 5 was common amongst patients with type 1 SMA, whereas most type 2 patients had SMN1 homozygous deletions of exons 7 and 8, but no deletions in the NAIP gene [89]. Here, SMN1 is linked to proximal spinal muscular atrophy.