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. The gene discussed is CR1; the disease is neoplasm.