The characterization of deletions in the SMN gene provides a helpful tool to confirm the diagnosis of spinal muscular atrophy (SMA). However, there may be homozygous deletions of the SMN gene in some unaffected siblings of SMA type II and III patients. We present two SMA families with affected siblings demonstrating a homozygous deletion of the SMN gene with extremely different phenotypes. We propose a preclinical category of an SMA patient with homozygous deletion of the SMN gene: those with minimal expression of the disease including cramps and EMG abnormalities that may develop the complete SMA phenotype in the future.