Some studies found that SMN-P levels in the peripheral blood relate to SMN copy numbers, the severity of denervation, and overall disease activity in mostly functionally more affected SMA patients [42–45] whereas different groups found neither a correlation between SMN-P blood levels and SMN copy numbers nor between SMN-P levels and muscle function and integrity [46, 43, 47]—or both findings at a time [43]. The gene discussed is SMN2; the disease is proximal spinal muscular atrophy.