The results of our validation study of a NBS assay (SALSA MC002 SMA Newborn Screen Assay, MRC-Holland, The Netherlands) demonstrated that the method exhibited 100% specificity and sensitivity in identifying individuals with SMA who had the most common underlying genetic defect—homozygous absence of the SMN1 gene, enabling identification of ~95–98% of SMA individuals. The gene discussed is SMN1; the disease is proximal spinal muscular atrophy.