Background: The Arias-Stella reaction (ASR) can mimic endometrial clear cell carcinoma (ECCC) in small biopsies, especially when drug or pregnancy history is unknown. A panel of immunohistochemical markers comprising napsin A, hepatocyte nuclear factor-1-beta (HNF-1β), estrogen and progesterone receptors (ER, PR) has been found useful in confirming a diagnosis of ECCC. However, the detailed characterization of how expression of this combination of markers in the ECCC mimics ASR has yet to be thoroughly evaluated. Design: The frequency and extent of napsin A, HNF-1β, ER, and PR expression in ASR were assessed in a large series. For napsin A, any cytoplasmic staining was considered positive while only nuclear staining was deemed to be positive for HNF-1β, ER, and PR. Immunohistochemical histoscores based on the intensity and extent of staining were calculated. Results: Forty cases were gestational and 10 were nongestational ASR. In 19 (38%), the reaction was extensive and involved >50% of the glands. A stromal decidual change was found in 31 (77.5%) of the gestational and 3 (30%) of the nongestational cases. Napsin A was positive in all gestational and 8 of 10 (80%) nongestational ASR. All ASR showed HNF-1β expression. ER expression was reduced in 37 (92.5%) and lost in 3 (7.5%) gestational ASR, and reduced in 9 (90%) and lost in 1 (10%) of nongestational ASR. None of the ASR in our series expressed PR. Conclusions: Naspin A and HNF-1β were frequently expressed in both gestational and nongestational ASR, and ER expression was usually either reduced or loss. Interpretation of these markers in small biopsies containing atypical clear cells should be made with caution.

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