The extensive (in terms of the number of evaluated patients), long-term (in terms of ability to prognose and predict the success/sufficiency of treatment), and standardized (in terms of the use of standardized and verified diagnostic tools) clinical trials, which would demonstrate the unequivocal suitability of using CA IX as a biomarker and its place in the diagnosis of BC (or other tumors), have not yet been performed. The gene discussed is CA9; the disease is breast cancer.