(A) HIV-HD NS subtype (case 26). RS cells show cytoplasmic and membranous staining of variable intensity for CD138/syn-1 in Bouin-fixed, paraffin-embedded tissue section. (B) HIV-HD MC subtype (case 6). A frozen section was tested by 2-color staining with BCL-6 MoAb and syn-1 MoAb. A BCL-6⁺(nuclear, blue) RS cell and syn-1⁺ (cytoplasmic and membranous, red) RS cells (arrows) are present. No coexpression of BCL-6 protein is detectable in syn-1⁺ RS cells. (C, D) HIV-HD LD subtype (case 25). (C) RS cells show EBV positivity by EBER in situ hybridization. Signal is present as dense brownish grains over the nuclei of RS cells. (D) Most RS cells display strong cytoplasmic staining for the EBV-encoded LMP1. (E) HIV-HD MC subtype (case 14). A frozen section was tested by 2-color staining with BCL-6 MoAb and LMP1 MoAb. An RS cell exhibits nuclear staining (blue) for BCL-6 (at left), whereas another RS cell shows cytoplasmic and membranous staining (reddish) for LMP1 (at right). No coexpression of both markers by the same tumor cell is detectable. (A, D) APAAP immunostaining, hematoxylin counterstain; (B, E) 2-color staining, no counterstain; (C) in situ hybridization, hematoxylin counterstain. Original magnification ×250, A-E.