Mutations in the SMN1 gene cause spinal muscular atrophy (SMA), a disorder characterised by progressive symmetric muscle weakness that can be complicated by other features including joint contractures, scoliosis, growth failure and restrictive lung disease. SMA is classified into clinical subtypes depending on severity and age of onset. Inheritance of SMA is autosomal recessive.

This test detects deletions of the SMN1 gene which cause more than 95% of cases of SMA. The result can have diagnostic and familial implications.


Interpretation of SMA test results will depend on the purpose of testing (diagnostic or carrier assessment), the patient’s family history and test results for other family members; however generally speaking test results are interpreted in the following way:

Diagnostic testing

Homozygous deletion (0 copies of SMN1 gene): confirms a diagnosis of SMA.

Heterozygous deletion (1 copy of SMN1 gene detected): is supportive of but does not confirm a diagnosis of SMA. Patients with this result and a clinical diagnosis of SMA can have rare sequence level mutations on the non-deleted copy of the SMN1 gene, and additional testing may be recommended.

No deletion (2 copies of SMN1 gene detected): a diagnosis of SMA is unlikely.

More Information:

ACOG recommends that screening for spinal muscular atrophy (SMA) be offered to all women who are considering or who are currently pregnant. SMA is a severe progressive neuromuscular disorder caused by loss of alpha motor neurons in the spinal cord, with the loss of muscle strength, leading to paralysis.

This disorder is common, with carrier frequencies in one study of 1/47 in Caucasians; 1/67 in Ashkenazi Jewish; 1/59 in Asian; 1/68 Hispanic; 1/52 Asian Indian; and 1/72 African American. In the overall US pan-ethnic population, the carrier frequency was 1/54 with a detection rate of over 90%.  SMA affects all population groups and is only second to cystic fibrosis as a cause of death from an autosomal recessive condition.