Spinal muscular atrophy is one of the most common autosomal recessive disorders, classified into three major clinical forms. It is caused mainly by deletions or gene conversions of the telomeric survival motor neuron gene (SMN1) on human chromosome 5. We have conducted molecular studies of the disorder in genetically isolated Romani (Gypsy) communities in Bulgaria and Hungary, where spinal muscular atrophy appears to have different prevalence and both mild and severe spinal muscular atrophy phenotypes have been diagnosed. We have observed three distinct genetic defects which, in different combinations, lead to different forms of the disease. The similar chromosomal background on which the different mutations occur suggests a common origin and founder effect, with rearrangements of a single ancestral chromosome resulting in a diversity of molecular defects.