ESR1 and breast cancer: This study demonstrated a significant difference (p < 0.001) in the comparison of pCR frequencies between subtypes (Luminal A and Luminal B HER2− (18.4%), Luminal B HER2+ (35.3%), HER2+ (57.4%), and triple negative (44.4%)), confirming that the chance of pCR is significantly higher in groups of breast cancer patients with higher tumor aggressiveness, quantified by HER2 expression or the absence of estrogen receptors as a sign of cellular undifferentiation.