Proliferative markers are not generally recommended as prognostic factors for clinical use in breast cancer due to lack of standardization in methodology.However, proliferation is driving several gene expression signatures emphasizing the need for a reliable proliferative marker for clinical use.Cyclin A is a protein expressed by cells during the s-phase and, consequently, a marker of proliferation.Previous studies have suggested that cyclin A also could be a prognostic marker.Objections being raised against the introduction ofcyclin A into the clinical setting have been: small studies,lack of cut-off value and unknown reproducibility.In several consecutive papers, we have addressed each of these issues in a methodological way.