At that time, OPN as a potential tumor marker in breast cancer fit into the category “+” on the Tumor Marker Utility Grading System (TMUGS) proposed by Hayes et al. [23], defined as “sufficient data are available to demonstrate that the marker correlates with the biologic process and/or biologic endpoint related to the use and that the marker results might affect favourable clinical outcome for that use. This evidence concerns the gene SPP1 and breast cancer.