Introduction: Bladder cancer is a common disease of the genitourinary tract for which the development of a noninvasive detection technique would have a significant impact on disease management. One of our previously identified bladder cancer-specific proteins, BLCA-4, appears to be associated with a "field effect" of the disease, and in clinical trials is able to separate individuals with bladder cancer from those without the disease with high sensitivity and specificity. The potential clinical utility of this marker has led to the analysis of its function in bladder cancer pathobiology. Experimental Design: To additionally analyze the specificity of this marker, the expression in the urine of a variety of benign urologic conditions was analyzed. After cloning the gene encoding BLCA-4, functional aspects of the protein were analyzed by overexpressing it in cell systems, as well as its interaction with other transcription factors and in gel mobility shift assays. Finally, to determine the timing of expression in relation to the observance of bladder cancer, an animal model of the disease was examined. Results: Expression of BLCA-4, the cDNA of which reveals that it is a novel member of the ETS transcription factor family, is not found in benign urologic conditions. Overexpression leads to increased growth rates of cells, and the protein interacts with other transcription factors. In vivo studies reveal that BLCA-4 expression occurs significantly before the observance of grossly visible tumors in an animal model of the disease. Conclusions: BLCA-4 is a bladder cancer marker that is highly specific and occurs early in the development of the disease. It appears to be a transcription factor that may play a role in the regulation of the gene expression in bladder cancer. BLCA-4 is a marker with significant clinical utility that may have an active role in the disease.