Klintman et al. conducted a gene expression study that included the quantification of 24 genes representing key biologic processes related to cancer in 56 pre- and post-NAC paired biopsies, and a total of 126 residual tumors, of which 66% were ER+, 52% PR+, 12% HER2+, 62% ER+/HER2−, and 26% ER−/HER2− [78]. This evidence concerns the gene ESR1 and cancer.