This is in consistence with several studies which have reported that loss of both copies of NAIP is not sufficient to cause the disease, as 1–2% of unaffected individuals and 2% of carrier parents, showed deleted NAIP gene in both chromosomes have found that in their SMA patients, deletion of NAIP gene was always associated with homozygous deletions of SMN1 gene. Here, SMN1 is linked to proximal spinal muscular atrophy.