YBX1 and breast cancer: Therefore, we evaluated the hypothesis that a subclassification of these carcinomas might be possible using Ki-67 proliferation index, cytokeratin 5/6 (CK5/6), cyclooxygenase-2 (COX-2), as well as Y-box binding protein 1 (YB-1) expression, for the latter two a role in breast cancer progression has been demonstrated previously [20,21].