Background: Breast cancer resistance protein (BCRP), or ATP-binding cassette subfamily G member 2 (ABCG2), is an ATP-binding cassette (ABC) transporter that mediates energy-dependent transport of substrate drugs out of the cell. Its overexpression may contribute to intrinsic drug resistance in vitro. However, the current literature has not yet clarified the clinical significance of BCRP/ABCG2 in invasive breast carcinoma. Objectives: The purpose of this study was to validate the expression of BCRP/ABCG2 in invasive breast carcinoma and its role in response to neoadjuvant chemotherapy. Methods: In this study, a pretherapeutic core biopsy was performed in 222 patients. BCRP/ABCG2 expression in carcinoma tissue was measured by immunohistochemistry. BCRP/ABCG2 expression correlations with clinicopathological features, molecular subtypes, and therapy response after neoadjuvant chemotherapy were investigated. Results: The results showed that BCRP/ABCG2 was expressed in different molecular subtypes. The proportions of patients with high BCRP/ABCG2 expression were similar in luminal A and luminal B tumors (luminal B, 80%; luminal A, 78%), compared with other molecular subtypes (triple-negative, 63%; HER2+, 58%, p = 0.05). BCRP/ABCG2 expression and the number of lymphatic metastases (p = 0.001) and tumor size (p = 0.011) demonstrated a statistically significant correlation. Low BCRP/ABCG2 expression was associated with an increased pathological complete response (pCR) rate of 38%, higher than the 19% in tumors with high BCRP/ABCG2 expression (p = 0.002). In multivariable analysis, BCRP/ABCG2 and hormone receptor (HR) expression were identified as independent risk factors of pCR (p = 0.003 and p = 0.013, respectively). Conclusions: BCRP/ ABCG2 is highly expressed in HR-positive breast cancer. High BCRP/ABCG2 expression is associated with lymphatic metastasis, tumor size, and poor pCR. BCRP/ABCG2 may be a novel potential biomarker that can predict clinical progression and therapy response after neoadjuvant chemotherapy. (C) 2021 S. Karger AG, Basel