Human pathology
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We report a case of localized Langerhans cell histiocytosis characterized by clonal aggregation of Langerhans cells in the thymus and identified with molecular genetic study. A 43-year-old Japanese woman was found to have an anterior mediastinal mass by radiologic studies. Laparoscopy-assisted biopsy was subsequently performed. ⋯ These Langerhans cells were immunohistochemically positive for S-100, CD1a, and CD207/langerin. Using allele-specific polymerase chain reaction and immunohistochemistry with mutation-specific antibody VE1, the BRAF V600E mutation was identified in aggregated Langerhans cells. At the medical follow-up, the thymic tumor had spontaneously regressed; however, identification of oncogenic BRAF mutation supports the neoplastic nature of the current case.