SMN1 and proximal spinal muscular atrophy: The remaining 5% of patients have compound heterozygous mutations, with 1 exon 7 deletion and 1 intragenic mutation on the other SMN1 allele.[18] According to the age of symptom onset and maximum motor function achieved, SMA can be divided into 5 subtypes: type 0 (death typically within 1 month of birth), type I (onset before 6 months, unable to sit independently), type II (onset after 6 months, unable to stand or walk independently), type III (onset after 18 months), and type IV (onset in adulthood).