SMN2 and spinal muscular atrophy, type 1: Interestingly, cases 13 and 14 both had two SMN2 copies and one mutation at the same site 3 nucleotides upstream of exon 7 in SMN1 (site c.835-3 of SMN1, case 13 with c.835-3C>A and case 14 with c.835-3C>T), but the clinical severity of the two varied greatly (case 13 with SMA type I and case 14 with SMA type IV); thus, splicing mutations caused by different base substitutions at the same site may have various effects on splicing.