To substantiate our prediction that ETBR is a druggable target for GBM and other cancer types, we used a standard viability assay to assess the cytotoxic effects of three clinically available endothelin receptor blockers—macitentan, bosentan, and ambrisentan—on primary GBM cells and GBM and breast cancer cell lines as compared with normal fibroblasts, endothelial and epithelial cells. The gene discussed is EDNRB; the disease is breast cancer.