EPHB4 and neoplasm: As it was considered unlikely that normal epithelial cells would be present within the peripheral blood [35,36], and malignant cells often continue to express markers that are characteristic of or specific for the normal tissue from which they originate [35], we used a combination of markers that were either expressed specifically in epithelial cells (Elf3/ESX and CK19) [19-24] or have been reported to be over-expressed in a high percentage of tumour cells from head and neck origins (EGFR and EphB4) [14-18].