Malignant mesothelioma (MM) is an aggressive tumor originating from serosal surfaces, mostly pleura [1]. Exposure to some carcinogens, especially asbestos, is a strong risk for MM development [2,3]. MM has an inadequate response to treatment because in most patients diagnosis is delayed. However, prognosis is better if recognized earlier. Therefore, it is crucial to diagnose MM early [4-6]. About 80% of MM cases are associated with asbestos exposure. MM incidence is high in Turkey, especially the Cappadocia region, where environmental asbestos is widely distributed [7]. It is thought that 20 years or more of asbestos exposure can increase the risk of MM development. Malignant mesothelioma, benign mesothelial neoplasia, and reactive mesothelial proliferation are lesions that are difficult to diagnose because they mimic each other cytomorphologically. Although the features supporting malignancy include clear cy-tological atypia, dense cell clusters, and necrosis, the most reliable diagnostic criterion is the presence of deep-tissue invasion [8]. MM diagnosis in cytologic materials is even more difficult because invasion is the only reliable standard. To date, an immunohistochemical marker that can reliably differentiate a reactive process from neoplastic mesothelioma does not exist [9,10]. Various immunohistochemical markers (epithelial membrane antigen, p53, p100, p-glycoprotein, desmin, etc.) are used together to reach a final decision [11-13]. BRCA1-associated protein 1 (BAP1) acts as a tumor suppressor and belongs to the family of high-risk cancer-related genes located at 3p21.1. It is associated with a high-risk cancer syndrome that includes malignancies such as malignant mesothelioma, uveal melanoma, cutaneous melanoma, atypical melanocytic tumor, and renal cell carcinoma (especially clear-cell type). BAP1 expression loss may be an indicator of malignancy in mesotheli