Although it is largely unknown how SMN-deficiency results in SMA, it is quite clear that reduced SMN in cell or animal models of SMA results in the reduction of U snRNP assembly [50, 52, 124–128], resulting in downregulation of assembly reactants—U snRNAs and Sm-protein mRNA—whilst maintaining a constant level of Sm-protein expression [9, 52, 107]. Here, SMN1 is linked to proximal spinal muscular atrophy.