To investigate this phenomenon in early- and late-stage mammary tumour development, anti-CD31 IHCs were performed on serial sections of 6-week-old mammary glands and mammary tumours excised at the ethical tumour volume end point, respectively, of MMTV-PyMT and MMTV-PyMTxHPSE−/− animals (Figure 4). The gene discussed is PECAM1; the disease is breast cancer.