Kennedy disease is one of a group of disorders referred to as spinal muscular atrophies. Kennedy disease is a progressive spinobulbar muscular atrophy (SBMA) that presents in individuals between 15 and 60 years of age. Kennedy disease involves instability of the trinucleotide repeats similar to fragile X, Huntington disease, Machado-Joseph disease, and myotonic dystrophy. In Kennedy disease it is believed that the androgen receptor gene is affected as a result of an abnormality to the X-chromosome ql2-q21. As such, the presence of Kennedy disease can be determined from diagnostic DNA testing. Because it is an X-linked recessive disorder, it occurs only in males but is carried by women. It is exceptionally rare: Only about 50 families are affected worldwide. There is no evidence to suggest a higher likelihood of occurrence based on ethnic group.