Tendl-Schulz KA, Rössler F, Wimmer P, et al. - In view of the essentiality of reliable determination of Ki67 labeling index (Ki67-LI) on core needle biopsy (CNB) for ascertaining breast cancer molecular subtype for therapy planning, researchers here examined how clinicopathological and sampling-associated factors influence agreement between molecular subtype and Ki67-LI between CNB and surgical resection (SR) specimens. In total, 484 pairs of CNB and SR specimens of invasive estrogen receptor (ER)–positive, human epidermal growth factor (HER2)–negative breast cancer were visually assessed for molecular subtype using Ki67-LI. As per findings, CNB had a sensitivity of 77.95% and a specificity of 80.97% for identifying luminal B tumors in CNB, compared with the final molecular subtype determination after surgery. They noted moderate correlation of Ki67-LI between CNB and SR. Tumor grade, immunohistochemical progesterone receptor (PR) and p53 expression were identified significantly influencing specificity and sensitivity for CNB to correctly define molecular subtype of tumors according to SR. No significant impact of agreement of molecular subtype on RFS and OS was noted. The recognized factors probably reflect intratumoral heterogeneity that might compromise acquiring a representative CNB. Results here challenge the robustness of a single CNB-driven estimation of Ki67-LI to recognize luminal B breast cancer of low (G1) or intermediate (G2) grade.