PMEL and melanoma: The most useful markers of histiocytes in RDD are positive for S-100 protein and CD68, and negative for CD1a.[14] In our case, immunohistochemical stains for monoclonal cytokeratin 7, EMA, MOP, calretinin, mesothelial cell, actin, desmin, HMB-45, melanoma, CD3, CD15, CD30, CD31, and ALK were all negative.