Spinal Muscular Atrophy: SMN1 Common Deletion Testing

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Condition Description

Spinal muscular atrophy (SMA) is the second most common lethal, autosomal recessive disorder in Caucasians. SMA is characterized by anterior horn cell degeneration which causes a symmetrical muscle weakness and wasting. Three types of SMA are described:

  • Type I(Werdnig-Hoffman disease): most severe form of SMA with an onset of symptoms before 6 months of age; affected individuals usually die by 2 years of age.
  • Type II(intermediate form): intermediate in severity between Types I and III, with an onset of symptoms between 6 and 18 months of age; death occurs after 2 years of age.
  • Type III(Wohlfart-Kugelberg-Welander disease): mildest form of childhood onset SMA, with symptoms beginning between 18 months and 17 years of age; affected individuals survive into adulthood.

All three SMA types are linked to chromosome 5q11.2-q13.3. A telomeric gene, known as the survival motor neuron (SMN1) gene, was found to be deleted in about 98% of patients. Point mutations are also known in this gene.

SMN1 is deleted in about 95% of individuals with SMA. This assay tests for the common SMN1 deletion only; other pathogenic variants will not be detected. Note that approximately 5-8% of individuals that are carriers of SMA carry two copies of SMN1 on one chromosome with a deletion on the second chromosome, and will not be detected with this assay. This assay will not report SMN2 copy number.

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Genes (1)

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SMN1 gene deletions were quantified by multiplex ligation polymerase chain reaction amplification (MLPA) of exons 7 and 8. Gene dosage ratios of SMN1 are calculated relative to the average of 16 reference loci and are expressed as gene dosage, and/or copy number, according to the SALSA protocol available from MRC Holland. Two copies of the SMN1 genes most often indicate normal (not affected) status and one copy of a deletion of this region most likely indicates carrier status.

Detection and Reference Range


Deletions of the SMN1 gene are found in approximately 95% of individuals with SMA. This carrier assay tests for the common SMN1 deletion only; other pathogenic variants will not be detected. Approximately 5-8% of carrier individuals will have a normal SMN1 copy number of two, but both copies will be on the same chromosome (in cis) with a deletion on the second chromosome. This assay will not detect these carrier individuals. SMN2 copy number is not assessed.

Reference Range

Qualitative assay.

Specimen Requirements

Type: Whole Blood

Specimen Requirements:

In EDTA (purple top) or ACD (yellow top) tube:
Infants (<2 years): 2-3 ml
Children (>2 years): 3-5 ml
Older Children & Adults: 5-10 ml

Specimen Collection and Shipping: Refrigerate until time of shipment. Ship sample within 5 days of collection at room temperature with overnight delivery.

Special Instructions

Diagnostic testing only. No prenatal or carrier testing available.

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