Bone marrow core biopsy showing infiltration by large lymphoid cells, extensively replacing normal hematopoietic elements. (A, hematoxylin and eosin, 200x). The lymphoid cells are large and have irregular nuclear contours with an increased nuclear-to-cytoplasmic ratio (B, hematoxylin and eosin, 400x) The lymphoid infiltrate is positive for PAX5 (C), showing aberrant overexpression of p53 (D) and a high Ki-67 proliferation index (E). (C-E; immunohistochemistry with hematoxylin counterstain, 200x). The bone marrow aspirate smear shows increased numbers of large, atypical lymphoid cells with prolymphocytoid morphology (F, Giemsa, 1,000x).