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Rare cases of seminoma show conspicuous signet ring cells. These cells are present singly or in small clusters in a background of typical seminoma. They contain a large cytoplasmic vacuole that compresses the nucleus into a peripheral crescentic shape. The vacuoles may be empty or contain glycogen that can be demonstrated by PAS positivity and removed by diastase pretreatment. The cells are negative for mucicarmine, cytokeratin AE1/AE3, and alpha fetoprotein whereas CD117 and OCT3/4 are positive. The presence of signet ring cells in seminomas may cause diagnostic confusion, especially in metastastic foci. Ref: Ulbright TM, Young RH. Seminoma with conspicuous signet ring cells : a rare, previously uncharacterized morphologic variant. Am J Surg Pathol. 2008 Aug;32(8):1175-81.