ESR1 and breast cancer: Ki67 IHC might be used for one of many possible applications, including for determination of breast cancer intrinsic subtype,21 use in IHC-based multiparameter assays to approximate results from gene expression assays such as the 21-gene recurrence score,14,15 and use in IHC-based prognostic models.22,23 Regardless, Ki67 is usually interpreted in the context of other clinicopathological parameters, such as tumor size, lymph node status and grade, or biomarkers, such as ER, PR and HER2 status.