Given this perspective, one of the most critically important mathematical variables related to cancer cell biology that can significantly determine the anti-neoplastic cytotoxicity of covalent immunochemotherapeutics like gemcitabine-[anti-HER2/neu], [19] [24] gemcitabine-[anti-EGFR], epirubicin-[anti-HER2/neu] [16] [18] [25] or epirubicin-[anti-EGFR], [16] is the expression density of “sites” on the external surface membrane of neoplastic cells utilized to facilitate the selective “targeted” delivery of chemotherapeutic moieties. This evidence concerns the gene ERBB2 and cancer.