MPO and neoplasm: FDCS tumor cells show an immunophenotype similar to that of normal FDC and express 1 or more of the markers CD21, CD23, CD35, R4/23, CNA42, Ki-M4P, EMA, and CD68.[19] They are typically negative for S-100, CD3, CD1a, CD30, CD34, SAM, CAM5.2, lysosomes and myeloperoxidase.[20,21] Positive IHC staining for CD21, CD35, and CD23 is particularly useful for the final diagnosis of FDCS, as it can distinguish FDCS from other spindle cell neoplasms.