NOTCH4 and carcinoma: We next compared the onset of palpable tumors in wild type and Notch4−/− mammary glands upon orthotopic injection of syngeneic primary MMTV-PyMT tumor cells wild type for Notch4. We designate the appearance of palpable tumors after injection of single cells dissociated from carcinoma tissues as tumor onset (hereafter referred to as such).