ESR1 and neoplasm: No significant differences in estimated tumor mutational burden (TMB) were observed according to HER2-low status, with a median TMB of 7.26 (0.76–85.94) for HER2-low and 7.60 (0.00–111.36) for HER2-0 samples (P = 0.24; stratified P by ER status = 0.28, stratified Kruskal-Wallis test).