Clark BZ, et al. –Given that biomarker analysis of invasive breast carcinoma can be useful for prognosis and prediction of response to adjuvant and neoadjuvant systemic therapies, researchers assessed breast cancer intratumoral heterogeneity in this quality assurance study. They performed comprehensive biomarker analysis of estrogen receptor (ER), progesterone receptor (PR), HER2, and Ki67 labeling index on each tissue block of 100 entirely submitted breast tumors in 99 patients. Findings suggested that intratumoral heterogeneity was higher for PR vs ER. Compared with associated ductal carcinoma in situ on surgical resection specimen, Ki67 labeling index was significantly higher in invasive carcinoma. In most cases, biomarker expression on diagnostic core biopsy or single tumor block is representative of breast carcinoma as a whole and is suitable for clinical decision-making.