To report the clinical, electrophysiological and genetic characteristics of a spinal muscular atrophy (SMA) family to explore the possible factors which may affect SMA phenotypes.

The clinical data of this family were collected, and the electromyography (EMG) and the survival motor neuron (SMN) gene testing were conducted.

The proband, male, 19 years old, presented with jumping difficulties and frequent falling down since 2 years old. At age 17, he presented weakness and atrophy, and the symptoms got worse progressively. He could not stand up after squating down and shaked hid hands involuntary when holding things. SMN gene test showed there was a homozygous deletion in exon 7 of SMN1 gene and a heterozygous deletion in exon 8. EMG suggested disease with anterior horn of spinal cord. The proband′s older sister got the same genotype in exons 7 and 8 of SMN1 gene as the proband with no symptoms. The other family members did not have any clinical symptoms. For the accessory examination, the proband′s mother and older sister got abnormal EMG.

As to SMA, the same genotype may have different clinical phenotypes. Several factors may affect SMA phenotype.And it is necessary to screen the SMA-related genes for SMA patients′ family members.

Spinal muscular atrophy; Phenotype; Copy number; Modifier gene