SMN gene

At least three genes have been reported to be associated
with the SMA type I phenotype on chromosome 5, namely
SMN, NAIP and p44. Diagnostic analysis in SMA patients is
restricted largely to analysis of the SMN gene. The SMN gene is
present in two copies, one centromeric (SMNC) and one
telomeric (SMNT). The absence of exons 7 and 8 in both
copies of the SMNT gene is a very reliable diagnostic test for
the majority of patients, confirming the clinical diagnosis of
SMA. Point mutations have been detected in affected
individuals who do not have homozygous deletions. The
PCR-based assay used for determining the presence or absence
of the SMNT gene is not able to detect individuals who are
heterozygous deletion carriers, and a gene dosage method of
analysis has been developed to improve carrier detection.