SMN1 and proximal spinal muscular atrophy: This dilemma was solved by studies that showed that the SMA phenotype is directly linked to the size of the genomic deletion.18 Researchers have discovered disease-modifier genes—the SMN2 and NAIP—that are located adjacent to SMN1 gene within a duplicative 5q region.19, 20 These observations have been demonstrated by studies conducted among Japanese, Vietnamese, Malaysian and Western ethnic populations.19–24Table 2 shows that when SMN1 deletion occurs concurrently with a single copy of SMN2, the patients always manifest the severe type of SMA, regardless of NAIP status.