SMN1 and proximal spinal muscular atrophy: As expected, and as shown in Fig. 5a, fibroblasts with wild-type SMN1 alleles expressed almost exclusively SMN exon 7 retaining transcript, fibroblasts from an SMA carrier (which lack one functional SMN1 allele) expressed both the exon 7 inclusion and exclusion variants, while fibroblasts from an SMA Type 1 patient (which lack both functional SMN1 alleles) predominantly expressed the exon 7 exclusion variant (encoded by SMN2).