Evaluation of staining for T-cell markers and TCA expression. (A) Indeterminate/negative staining showing lack of definitive membrane accentuation on the HRS cells in areas rosetted by T cells. Free area of HRS cell membrane (denoted with white arrow) opposite to areas of partial rosetting lacks distinctive membrane staining (CD4 immunoperoxidase, ×400). (B) Indeterminate staining showing complete rosetting by CD4-positive T cells without apparent accentuation on the HRS cells (CD4 immunoperoxidase, ×400). (C) Positive staining showing clusters of HRS cells with definitive but incomplete membrane staining. Typically, CD2 and CD7 stains showed areas with frequent incomplete staining patterns (CD2 immunoperoxidase, ×200). Cases with such staining patterns often showed more complete staining pattern in other areas of the same cases or displayed convincing positivity for another T-cell marker. (D) Definitive, complete circumferential membrane staining in clusters of HRS cells (CD2 immunoperoxidase, ×200). (E) Membrane and cytoplasmic Golgi staining for CD8 (immunoperoxidase, ×400). (F) Strong and diffuse cytoplasmic staining for perforin. This case was unique in that almost all HRS cells showed uniform, strong expression of perforin (perforin immunoperoxidase, ×400).