Frequent DNA variant in exon 2a of the survival motor neuron gene (SMN): a further possibility for distinguishing the two copies of the gene.

An intragenic single-strand conformation polymorphism (SSCP) variant in exon 2a of the survival motor neuron gene (SMN) has been identified. The SSCP band shift is caused by a silent mutation (AGC-->AGT) at codon 28, which is the first codon of exon 2a. Five exchanges of base pairs at the 3'-end of the gene have been described that allow the two copies of SMN (telSMN and cenSMN) to be distinguished, whereas no DNA variant has been found at the 5'-end. The new DNA variant belongs to cenSMN and may be important for the assignment of point mutations to one of the two copies of SMN in spinal muscular atrophy (SMA) patients. The frequency of this variant is lower in SMA patients (10%) than in controls (24%).