genetic counselling carrier and normal populations: implication for deletion and duplication in SMN1 Prevalence of

defines several degrees of severity in the SMA phenotype,depending on the age of onset and motor development mile-stones. Type I SMA, Werdnig-Hoffmann I disease, is the mostsevere form with onset within 6 months of birth. Patients areunabletositupandhaveseriousrespiratorydysfunction.TypeII SMA is the intermediate form with onset within the first 2years; children can sit up but are unable to walk. The clinicalcourse is variable. Type III (also called Kugelberg-Welanderdisease) begins after 2 years of age and usually has a chronicevolution. Children can stand and walk unaided at least ininfancy. Adult form (type IV) is the mildest, with onset after30 years of age; few cases have been reported and itsprevalence is not accurately known.Spinal muscular atrophy is linked to locus 5q13 in morethan 95% of patients.