Spinal muscular atrophies (SMA) are a group of motor neuron diseases characterized by degeneration of anterior horn cells of the spinal cord and by muscular atrophy. Childhood-onset SMA is one of the most frequent autosomal recessive diseases and a leading cause of infant mortality. The underlying biochemical defect of SMA is unknown. Recently two genes have been isolated from the critical region at 5q13, the survival motor neuron (SMN) gene and the neuronal apoptosis inhibitor protein (NAIP) gene. Both genes are frequently deleted in SMA patients. NAIP is deleted in at least 45% of severely affected patients but less frequently in the milder forms. Homozygous deletions of exon 7 of SMN are found in approximately 95% of patients independently of clinical severity. A few point mutations and microdeletions in SMN have also been reported. This high frequency of deletions makes SMN analysis an important molecular diagnostic tool for childhood-onset SMA and greatly facilitates prenatal diagnosis. SMN analysis has also proven useful for the diagnosis of adult-onset SMA and variant forms. Although questions such as phenotype-genotype correlation must still be solved, the isolation of SMN and adjacent genes constitutes an important step towards the understanding of the molecular basis of the disease.