One example of how a CNV can ameliorate the clinical phenotype is spinal muscular atrophy where an increased copy number of the SMN2 gene can greatly reduce the severity of the disease caused by the homozygous deletion of the SMN1 gene, because the SMN2 gene, which lacks a splicing enhancer, can nevertheless generate some functional product thereby compensating functionally in a copy number-dependent fashion for the loss of the SMN1 gene (Vitali et al. 1999; Harada et al. 2002; Wirth et al. 2006). This evidence concerns the gene SMN1 and proximal spinal muscular atrophy.