Figure 1

Schematic of SMN1 and SMN2 alleles on chromosome 5q13. (a) Wild type with one copy of SMN1 and SMN2 on each chromosome 5 (1 + 1). (b) SMA carrier with one copy of SMN1 on one chromosome 5 and loss of SMN1 on the other chromosome (1 + 0). (c) Duplication with two copies of SMN1 on one chromosome 5 and one copy on the other chromosome (2 + 1). (d) SMA silent carrier with two copies of SMN1 on one chromosome 5 and loss of SMN1 on the other chromosome (2 + 0).