However, CD99 has been implicated as playing dual roles in tumors, as an oncogene (glioma, Ewing’s sarcoma, lymphoma/leukemia, melanoma, and breast cancer) or an oncosuppressor (osteosarcoma, Hodgkin’s lymphoma, and cancers of the stomach, pancreas, and bladder), as recently reviewed [13]. This evidence concerns the gene CD99 and osteosarcoma.