Compositions and methods are provided for diagnosis, prognosis and treatment of AR-related diseases, such as prostate diseases, such as prostate cancer, breast cancer, and many other diseases. In particular, a novel and clinically relevant mutation at position 876 of the androgen receptor (AR) has been identified. Drug sensitivity can be predicted and therapeutic regimens can be planned on the basis of the presence or absence of this mutation. Polypeptides comprising, antibodies to, and polynucleotides encoding the mutant AR can be used to identity novel treatments. A double mutation in AR at positions 741 and 877 is also shown to be useful for patient stratification.
