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Diagnostic Services

Genetics Lab


Prader-Willi Syndrome - DNA Methylation

Prader-Willi syndrome is associated with neonatal hypotonia and feeding difficulties, changing to excessive eating later in infancy and childhood. Motor, cognitive, and language development are typically delayed. Prader-Willi syndrome is caused by the absence of the paternally-inherited criticial region of chromosome 15.

For diagnosis, DNA methylation study of the PWS/AS Critical Region is the recommended method. If the methylation pattern is characteristic of maternal inheritance, this is diagnostic for Prader-Willi syndrome. Maternal inheritance of the PWS/AS critical region can be caused by deletions of the paternal allele, receiving two copies of chromosome 15 from the mother (uniparental disomy) or defects in the imprinting process.

If the methylation test is positive, a FISH (fluorescence in situ hybridization) test can be used to distinguish between uniparental disomy and a deletion.

99% of cases of Prader-Willi syndrome will be diagnosed by DNA methylation studies.

Test Name: DNA Methylation study - Prader Willi syndrome
Test Code:
Test Description:
Methylation-specific PCR assay of SNRPN alleles.
This test is indicated for individuals with:
-neonatal hypotonia and feeding difficulties
-persistent hypotonia, history of poor suck, developmental delay
-history of any of the above in addition to excessive eating
Related Tests: Fluorescence in sity hybridization (FISH) ONLY to R/O Prader-Willi Syndrome  
This test is used as a follow-up test to a positive Prader-Willi syndrome DNA methylation study.

Clinical Links: Prader-Willi Syndrome GeneReview
Mon - Fri
Turnaround Time:
2 weeks
Methylation sensitive PCR
Reference Range:
Interpretive report provided
Consent Form:
Sample Requirements: Type
Whole blood: 1-3 mL in EDTA (purple top) or ACD (yellow top)
DNA: 5-20ug extracted DNA

Refrigerate blood samples which will not be immediately shipped.

Room temperature via overnight shipping.
Samples Received:
Monday - Friday 8am - 5pm PST
Special Considerations:
Please e-mail Lab Client Services ( or call at 206-987-2617 with any billing questions
CPT Code:
Please e-mail Lab Client Services ( or call at 206-987-2617 with any billing questions