ERBB2 and breast cancer: Within BrM, TMB did not significantly differ by intrinsic subtype, but did vary by clinical subtype in FFPE BrM, with HR + HER2− exhibiting higher average TMB than other subtypes, a feature which supports some prior reports of higher TMB in HR + HER2− and HR-HER2− metastatic BC compared to early stage or primary BC [5, 26], though this has not been found across every study [2].