- Cells must express CD2. CD2 expression will be verified by immunohistochemistry in the Laboratory of Pathology at the NIH. At least 30% of tumor cells must be CD2 positive for the patient to be eligible for the study by immunohistochemistry. CD2 staining will be performed on existing tissue blocks and on fresh tumor tissue if a biopsy is performed. It is expected that the majority of patients will have CD2 expression evaluated by flow cytometry and the majority of cells will express the marker.