Although the small amount of full-length SMN protein derived from SMN2 is not sufficient to fully compensate for loss of SMN1, it is essential for viability in the absence of SMN1, and is an important disease modifier: in both SMA patients and mouse models, there is an inverse relationship between SMN2 copy number and disease severity [17,18]. Here, SMN2 is linked to proximal spinal muscular atrophy.