In a comprehensive study, Chandarlapaty et al. performed allele-specific ddPCR of ESR1 mutations Y537S and D538G, which lead to ligand-independent constitutive activation of ERα in blood samples from 541 ER+/HER2− breast cancer patients with metastatic diseases that all had undergone AI therapy in different settings (BOLERO-2, NCT00863655) [55]. The gene discussed is ESR1; the disease is breast cancer.