Tumor heterogeneity plays a critical role in oncology research and clinical practice, providing valuable insights into disease progression, therapeutic selection, and clinical outcomes of patients; for example, breast cancer subtype-based systemic therapy selection,88 EGFR mutations and anti-EGFR therapies in lung cancer,84 stem cell gene expression programs in leukemia,23 and intra-tumor heterogeneity levels and cancer evolution in human glioblastoma.78 The gene discussed is EGFR; the disease is breast cancer.