CD81 and cancer: The remaining three malignant tumors corresponded to rare neoplasms and included i) one CD56++ CD271+ CD9++ CD81++ CD10+ pseudopapillary tumor of the pancreas (Frantz tumor) (n = 1) and ii) two CD56++ CD81+ CD90− CD271++ undifferentiated malignant tumors that could not be further classified by histopathology and IHC; in one of these two later tumors, neoplastic cells co-expressed CD99+ and GD2+, while in the other sample, they were CD99− GD2− CD10+ CD57+ CD9+ nuMyoD1− numyogenin−.