X-linked recessive bulbospinal neuronopathy (SBMA) is an adult onset motor neuronopathy with androgen receptor (AR) gene mutation of expanded CAG repeat size in the first exon. The size of CAG repeats in the AR gene is one of the determinant factors of the severity and progression rate of SBMA phenotypes, but the meiotic and somatic instability of CAG repeats is far more stable as compared with other diseases caused by trinucleotide repeat expansions such as HD, DRPLA, MJD and SCA1. Several evidences suggest that aberrant transcriptional activity of androgen through mutant AR is related to the pathogenic mechanism of this disease.