Adnexal tumors with follicular differentiation in the breast parenchyma are rarely encountered. The authors present a unique case arising in a 64-year-old woman, in whom they observed composite differentiation toward follicular germinative cells of the hair follicle with focal areas of outer root sheath differentiation and pilar-type keratinization. The histogenesis of this tumor is analyzed in light of the peculiar pathological, immunohistochemical, and molecular genetic findings.
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