Smith-Magenis syndrome chromosomes. Fluorescence in situ hybridisation (FISH) micrograph showing a deletion on chromosome 17. This is the cause of Smith-Magenis syndrome. In FISH, chromosomes are dyed to selectively highlight individual genes. On the normal chromosome at left, the normal gene is present, and shows up as two green dots. On the chromosome at right, these dots are absent. This is caused by a mutation. Smith-Magenis syndrome causes mental retardation and multiple compul- sions, such as picking at nails, and inserting foreign objects into body orifices. Treatment is based on the symptoms. Magnification unknown.