The retention of CD133 expression by the CSF-1Rhigh cell population brings up the intriguing possibility in hemangiosarcomas that the CSF-1Rhigh cell population possesses the potential for differentiation into CSF-1Rlow cells and may be responsible for maintaining tumor growth autocrine or paracrine loops since we previously determined that the ligands for CSF-1R, CSF-1 and IL-34, are expressed by these and other canine hemangiosarcoma cell lines (B. H. Gorden, unpublished data). This evidence concerns the gene PROM1 and angiosarcoma.