We have therefore aimed to (i) describe the relationship of cut-points for ER with IHC or qPCR with Ki67 response to short-term treatment of primary breast cancers with a non-steroidal aromatase inhibitor, (ii) determine whether cut-points for IHC or qPCR were related to Ki67 response more closely than the other or whether the addition of qPCR to IHC could improve that relationship and (iii) to determine whether these relationships differed according to HER2 and PgR status of the tumours. This evidence concerns the gene ESR1 and breast cancer.