Compositions and methods for diagnosing and treating breast cancer, including metastatic breast cancer, are provided. In one aspect, a diagnostic composition comprising a reagent which is capable of specifically complexing with, or identifying, GABRA3 is provided. In another aspect, a method of detecting breast cancer in a subject comprising measuring the level of GABRA3 in a biological sample from the subject is provided. In yet another aspect, a method of treating breast cancer is provided, the method comprising: measuring the level of GABRA3 in a biological sample from a subject and treating the subject with a reagent that inhibits the action of GABA when GABRA3 is detected in the sample or when there is an increase in the level of GABRA3 in the sample as compared to a control sample from a healthy subject.
