SMN1 and proximal spinal muscular atrophy: Patients survive due to low amounts of the SMN protein being translated from transcripts of the orthologous SMN2 gene, a nearly identical copy of SMN1. Hence, the SMA disease severity is mainly determined by the level of compensatory transcription of available SMN2 copies and is clinically classified by the highest motor milestone achieved and disease-onset as type I (infantile-onset, sitting is not achieved), type II (late-onset, walking is not achieved), and type III (late-onset, walking is achieved)4.