Abstract: The present invention relates to a gene selectively expressed in malignant cells and associated with the development of androgen resistance in prostate cancer. Levels of the gene, termed “PAR” (for Prostate Androgen Regulated), its RNA transcript, and its protein product are all present at increased levels in malignant cells, such as breast cancer and prostate cancer cells. The present invention provides for PAR nucleic acid molecules and proteins, for antibodies that specifically bind to PAR proteins, and to methods for diagnosing and treating cancers that utilize such molecules.