Although, they have high concentrations on ~70–90% EC-like and non-EC-like GBM cells, their high GBM heterogeneity (QE = ~1.0) and high concentration on healthy tissue (2–2,000 × 103 EGFR/fibroblast or epithelial cell; 2.5 × 104 IGFR/NIH 3T3 mouse fibroblasts (Sorkin and Duex, 2010; Brennan et al., 2013; Weddell and Imoukhuede, 2017) may lower their targeting specificity, resulting in lower drug efficacy (Wheeler et al., 2010). The gene discussed is IGF1R; the disease is glioblastoma.