2.1.1.4 At least 10% of the cells obtained from lymph node, or extranodal sites must react with anti-cluster of differentiation 25 (CD25) (anti-Tac) on immunofluorescent or immunoperoxidase staining. Because of the high frequency of CD25 positivity of the infiltrating Tcells in HL tumors, patients with CD25-positive infiltrating T cells will be eligible even if their Hodgkin's (Reed-Sternberg) cells are CD25-negative.