Diagnosis of SMA type I

SMA type I was diagnosed according to age of onset and maximum motor milestones achieved and copy numbers of SMN2.¹³ Molecular genetic testing, in particular targeted mutation analysis, consisted of either single gene or multi-gene panel testing. Multiplex ligation-dependent probe amplification (MLPA) was used to quantify variations in SMN1 and SMN2 copy numbers. MLPA was performed using SALSA MLPA Probemix P021 SMA from MRC-Holland (NL): this kit contains specific probes for exons 7 and 8 of SMN1 and SMN2, neuronal apoptosis inhibitory protein, and additional gene sequences within the 5q chromosome region. Homozygous deletions of exons 7 and 8 of SMN1 on the 5q13.2 locus were identified in the included patients.