Expansion of the long (CAG; glutamine)_n repeat in the first exon of the X–linked human androgen receptor gene (hAR) causes spinal and bulbar muscular atrophy, frequently in association with mild androgen insensitivity. The relevant normal motor neurons are preferentially stimulated by androgen, however no motor neuron disorder occurs with any other known AR mutation, including those that cause complete androgen insensitivity. We have found that a polyglutamine (Gln) expanded AR transactivates an androgen–responsive reporter gene subnormally. Other groups have reported that a poly Gln–deleted AR transactivates normally. A parsimonious interpretation of all these facts is that poly Gln expansion causes the AR to lose a function that is necessary for full androgen sensitivity and to gain a function that is selectively motor neuronotoxic.