Stable NIK expression can be detected in both HL cell lines and primary HL biopsies. (A) Row 1: Normal human tonsil. NIK expression is restricted to the germinal center (GC), particularly centroblasts appearing positive, whereas the mantle zone cells (MZ) are negative. Rows 2-3: High magnification of A1 showing a cytosolic granular staining in centroblasts. (B) Rows 1-3: Lymph node biopsies of 3 HL patients. Arrows indicate the granular cytosolic NIK staining in RS cells. (C) Rows 1-3: HL cell lines L1236 (C1) and U-H01 (C2) with NIK expression in the cytosol of tumor cells. (C3) SUDH-4, a DLBCL lymphoma cell line, is negative.