Objective:Kaposi's sarcoma (KS) is an acquired immunodeficiency syndrome (AIDS)-defining neoplasm histologically characterized by proliferation of spindle cells, inflammatory cells, and abundant neovascularization. When the malignant cell line KSY-1 derived from an AIDS-KS tumor is transplanted subcutaneously into nude mice, prominent neovascular features develop. Using this mouse model of neoplastic KS, we set out to determine, usingc-ets 1markers specific for mouse or human tissues, whether vascular growth and inflammatory infiltrate induced by the transplanted KSY-1 cells is of host cell or transplant origin.Study Design/Methods:KS tumors were induced by subcutaneous inoculation of 5 × 106KSY-1 cells/200 &mgr;L in immunodeficient mice, and species-specific mouse and human riboprobes of thec-ets 1protooncogene were used for in situ hybridization to define cell of origin.Results:Five different tumors were examined. Tissue sections from all cases were hybridized with radiolabeled riboprobes for the presence of both mouse and human c-ets 1 mRNA. Tumor cells were labeled with the human c-ets 1 probe, whereas neovascular and inflammatory tissues were of mouse origin.Conclusions:The finding that vascular but not tumor cells are of host origin supports the model of tumor-induced vascularization via a mechanism of tumor cell-derived cytokinemedicated pathogenesis.Journal of Human Virology 1999;2:315-317 © Lippincott Williams & Wilkins, Inc.