In experiments using other tumor types which do not produce CSF1 (synovial sarcoma, clear cell sarcoma, and myxoid liposarcoma), no host F4/80+ macrophage infiltrates were seen (Cheng and Nielsen, unpublished observations), supporting that their presence in PVNS xenografts indeed occurs as a consequence of tumor CSF1 expression rather than as a reaction to the implantation procedure. The gene discussed is CSF1; the disease is clear cell sarcoma.