Thirty-seven CAG repeats in the androgen receptor gene in two healthy individuals.

X-linked recessive spinobulbar muscular atrophy (SBMA) is an adult-onset X-linked neurodegenerative disease, characterised by muscular atrophy, bulbar symptoms and endocrinological disturbances. SBMA is caused by the expansion of a CAG repeat in the androgen receptor gene. The maximum number of CAG repeats found in a healthy person is 35 while the minimum number of repeats found in SBMA patients is 38. We have identified a 46-year-old man from an SBMA family with 37 CAG repeats who until now is clinically unaffected. Interestingly, his 85-year-old mother who has the genotype 37/51 CAG repeats is clinically unaffected as well. These results suggest an exactly defined border between normal and disease alleles.