CA9 and breast cancer: Analysis of metabolism-related protein expression according to metastatic site in metastatic breast cancer revealed site-specific expression patterns of glycolysis-related proteins (Glut-1, hexokinase II, CAIX, and MCT4), with higher expression levels seen in brain metastasis than in bone or liver metastasis (p < 0.001, 0.001, 0.009, and <0.001, respectively).