Case 4: synchronous FL and interdigitating cell sarcoma (IDCS). (A) An inguinal lymph node biopsy showed both FL, grade 1, and IDCS. The latter process occupied the majority of the lymph node biopsy, with only focal areas of FL. Magnification ×200. (B) The IDCS showed strong staining for S100 protein. Magnification ×200. (C) Cells of IDCS showed nuclear staining for CEBP beta, negative in FL lymphocytes. Magnification ×400. (D) The IDCS showed splitting of the BCL2 signals (arrow), consistent with presence of the t(14;18), using a breakapart fluorescence in situ hybridization (FISH) probe. A control sample shows no splitting of the BCL2 signals. Magnification ×1250.