The Survival of Motor Neuron 2 (SMN2) gene is homologous to the SMN1 gene; both of which are located on a large region of inverted repeat on chromosome 5q. The SMN2 gene lies towards the centromeric side of this repeat, as opposed to the SMN1 gene, which lies on the telomeric side. The SMN1 protein is part of a complex which plays an important role in the spliceosomal snRNP assembly and thereby is integral for the process of pre-mRNA splicing that takes place in the nucleus. This function of the protein is especially evident in the motor neuronal cells, where a loss of function of the protein affects the development of axons and dendrites from these neurons. The SMN protein, therefore, plays an important role in the transmission of neuronal impulses. Both the SMN proteins have the same function. However, the SMN2 protein is produced in much smaller amounts than the SMN1 protein, thereby decreasing the relative significance of the SMN2 gene. The effect of the SMN2 gene, therefore, is dependent on its copy number.
In patients with Spinal Muscular Atrophy (SMA), the SMN1 gene may be affected. In such cases, higher copy numbers of the SMN2 gene may help in ameliorating the severity of the disease condition. Patients with defective copies of the SMA1 gene, who also have up to three copies of the functional SMA2 gene have much milder symptoms of SMA which begin later in life than those who have only the regular one or two copies of the SMA2 gene. Other studies also point out to an increased risk for the development of Amyotrophic Lateral Sclerosis (ALS) in individuals with reduced copies of the SMN2 gene.