PTPRC and lymphoma: However, this tumor was negative for CK7, CK20, CK5/6, p40 (Cytokeratin 5/6 and p40 were used to exclude squamous differentiation), S100 (S100 was used to exclude possible metastatic melanoma due to high grade tumor morphology) and CD45 (CD45 wasused to exclude hematopoietic origin as with this small blue cell morphology lymphoma was in the differential diagnosis).