KDR and neoplasm: Tumor biomarkers include proteins or receptors that are abnormally expressed in the plasma membrane of the tumor cell, e.g., the vascular endothelial growth factor receptor (VEGFR), the epidermal growth factor receptors (EGFR and HER2), interleukin-4 (IL-4), transferrin, and the folate receptor (FOLR) [52].