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The marrow shows a polymorphous infiltrate composed of a mixture of small lymphocytes, eosinophils, plasma cells, and histiocytes. Scattered within this background are characteristic Reed–Sternberg cells and their variants, which are large atypical cells with abundant cytoplasm and bilobed or multinucleated nuclei containing prominent eosinophilic nucleoli (“owl’s eye” appearance). The background may show varying degrees of fibrosis. The normal marrow architecture is often partially or diffusely effaced by the infiltrate. The presence of Reed–Sternberg cells in an appropriate reactive cellular milieu is the key diagnostic feature of marrow involvement by Hodgkin lymphoma.