SMN1 and proximal spinal muscular atrophy: As mentioned earlier, approximately 5% of all cases of SMA linked to 5q13 result from intragenic mutations within SMN1 as opposed to the loss of SMN1. Table 1 provides a list of the currently known SMA-associated intragenic mutations in SMN1. The SMA-associated intragenic mutations located within the exons can be either missense, nonsense or frameshift mutations.