Variation in tumor response definitions, NACT protocols (e.g., schemes, doses, and treatment cycles), tumor characteristics (e.g., size, ER, progesterone receptor, and HER2 status), and subtype distribution also contributes to the heterogeneity in the studies, which may be responsible for fluctuations in the pCR rates (13.2% to approx. 52.0%). The gene discussed is ESR1; the disease is neoplasm.