SMN1 and proximal spinal muscular atrophy: Most SMA patients harbor genetic mutations in SMN1 and have more than two copies of SMN2. An increased copy number of SMN2 is reported to be strongly associated with an SMA phenotype, and the relationship between the SMN2 gene copy number and the amount of SMN protein in peripheral blood nuclear cells (PBCs) has been analyzed using a sandwich enzyme-linked immunosorbent assay (ELISA) [7] and electrochemiluminescence immunoassay (ECLIA) [8].