MKI67 and breast cancer: Analysis of clinical markers of human breast cancer revealed that the tumours arising in the EdarTg951/951 mice lacked Oestrogen Receptor (ER) expression (Fig. 3D) were highly proliferative, with many cells staining positively for the proliferation marker Ki67 (Fig. 3E), and contained large areas of apoptosis, marked by cleaved Caspase-3 staining (Fig. 3F).