Novel co-modulator proteins, designated ARAP3 polypeptides, for nuclear receptors, especially the androgen receptor, are described, which are useful in methods of detecting and treating steroid hormone-dependent diseases that are due to a deficiency of these co-modulator proteins. By measuring the levels of ARAP3 polypeptide expression in tissue samples from an individual a steroid hormone-dependent disease, such as breast cancer, can be detected. The levels of ARAP3 polypeptide expression in the tissue samples can be measured by immunohistochemistry methods, by ELISA methods, by radioimmuno tests, by Northern blot techniques, or by Western blot techniques A method of treating steroid hormone-dependent diseases due to a deficiency of an ARAP3 co-modulator protein in various tissues of an individual includes incorporating ARAP3 co-modulator protein in the tissues using a vector.