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Metaplastic breast carcinoma (MBC) and gynecologic carcinosarcoma (GCS) are rare, clinically aggressive cancers that demonstrate epithelial components and mesenchymal or sarcomatoid components. In this study, we assessed PD-L1 expression and tumor-infiltrating lymphocytes (TILs) in MBC and GCS. Overall, PD-L1 positivity using the SP142 clone was seen in 50 % of MBC and 51.9 % of GCS cases, with PD-L1 expression in tumor cells significantly higher in MBC cases (p = 0.034), and PD-L1 expression in immune cells similar in MBC and GCS cases. PD-L1 expression was significantly higher in epithelial components than in mesenchymal components in both MBC and GCS cases (p = 0.0005). TILs were low in the majority of MBC and GCS cases (≥ 10 %) and generally correlated with PD-L1 expression; however, many PD-L1 positive cases with low TILs were seen. PD-L1 expression using the 22C3 clone was additionally assessed, with positivity seen in 62.9 % of MBC cases and 30 % of GCS cases. Concordance between SP142 and 22C3 results was seen in 62.5 % of MBC cases and 80 % of GCS cases. Overall, our findings suggest that a subset of MBC and GCS cases may benefit from immune checkpoint inhibitor therapy. Our findings also illustrate unique aspects of PD-L1 expression patterns in these tumors which may harbor additional prognostic and therapeutic significance., Competing Interests: Declaration of competing interest None., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)