Mortality from breast cancer is primarily due to the formation of distant metastases. However, the molecular properties of primary breast tumors that dictate metastattic potential are poorly understood. Such knowledge would identify patients with high risk for systemic disease and would allow for the development of specific therapeutic strategies with significant potential for decreasing mortality. We have recently demonstrated that primary breast tumors with high metastatic potential express genes induced by hypoxia. One of these genes codes for a protein called carbonic Anhydrase IX ( CAIX), which we have now shown to be vital for the survival of metastatic breast tumor cells. Genetic ablation of this protein in a mouse model of breast cancer results in dramatic regression of breast tumors and complete inhibition of distant metastasis. Most significantly, ananlysis of 4,000 human breast cancer samples demonstrates that patients with tumors that express high levels of CAIX protein, have a poor survival outcome and high potential of distant metastases. In this aplication we wish to extend this exciting findings and propose to: 1) validate the vital role of CAIX in breast cancer metastasis by examining additional breast cancer models and perform detailed structure-function analysis of CAIX protein to understand its role in protecting hypoxic tumor cells; 2) develop novel imaging reagents as diagnostic an prognostic tools for identifying and following therapeutic response of agressive breast cancers; and 3) develop novel antibody and small molecule compounds that inhibit CAIX function for use as cancer therapeutics therapeutics. Our studies should result in the ability to identify breast cancer patients with high risk of metastasis, and in the development of novel prognostic and therapeutic reagents for aggresive breast cancers