SMN1 and proximal spinal muscular atrophy: On the other hand, in SMA subjects, in which SMN1 is unproductive, all the reads associated by the mapper with a probabilistic calculation to SMN1 actually come from SMN2. MultiDEA reported this ambiguity in the mapping, highlighting that almost 100% of the reads that mapped to one of these two genes also mapped identically to the other, while there was no ambiguity in these reads compared to other genes (no other genes showed significant portions in common with SMN1 and SMN2).