Transcription factor expression in histiocytic sarcomas. (A,B) Staining for CEBP beta in cases 5 and 1. Tumor cells show strong nuclear expression. Note negative blood vessel in panel B. (C,D) Staining for PU.1 in cases 4 and 1. Neoplastic cells (C) show sinusoidal localization. Note negative adjacent lymphocytes. Histiocytic cells have pleomorphic nuclei and prominent nucleoli. (E,F) Staining for PAX5 in cases 8 and 1. No nuclear staining is observed. Follicular lymphomas demonstrated normal expression of PAX5 (not shown). Original magnification for all figures ×400 (hematoxylin counterstain).