SMN1 and proximal spinal muscular atrophy: Nevertheless, approximately 10% of the mRNA produced from SMN2 is correctly spliced and produces full-length SMN protein, although the amount produced is insufficient to fully compensate for loss of SMN1. Because the SMN locus is unstable, the copy number of SMN2 can vary between individuals, with more copies (3–4) generally associated with milder forms of SMA (4,5,8).