SMN1 and proximal spinal muscular atrophy: Molecular diagnosis of SMA— i.e., loss of SMN1—has historically been made using a polymerase chain reaction (PCR)-based assay followed by digestion of the PCR product with specific restriction endonucleases (PCR-RFLP; Lefebvre et al., 1995; van der Steege et al., 1995).