For example, the presence of a dominant modifier DFNM1 leads to normal hearing in an individual homozygous for the DFNB26 mutation [34]; high expression of actin-binding protein plastin 3 (PLS3) protects individuals carrying homozygous SMN1 deletions from developing spinal muscular atrophy (SMA) [35]; and among the two siblings affected by autosomal recessive polycystic kidney disease (ARPCKD), one died at 18 hr but the other still had no symptom when presented at 16 [36]. The gene discussed is SMN1; the disease is proximal spinal muscular atrophy.