The purpose of the present study was to search for the presence of a tumor-initiating stem cell population in renal carcinomas. Based on the recent identification of mesenchymal stem cells in normal kidneys, we sorted cells expressing the mesenchymal stem cell marker CD105 from 5 human renal carcinomas. Because the CD105⁺ but not the CD105^- population showed enhanced tumorigenicity when injected in severely compromised immunodeficient (SCID) mice, we cloned and characterized CD105⁺ cells and evaluated their stemness, differentiative ability, and serial tumor generation. Characterization of the phenotype of CD105⁺ clones revealed several stem cell properties: 1) clonogenic ability, 2) expression of nestin, Nanog, Oct4 stem cell markers, and lack of differentiative epithelial markers, 3) ability to grow in nonadhesive spheroids, 4) in vitro differentiation into epithelial and endothelial cell types, and 5) generation in vivo of serially transplantable carcinomas containing an undifferentiated CD105⁺ tumorigenic and a differentiated CD105^- nontumorigenic population. In addition, some vessels present in carcinomas generated from CD105⁺ clones were of human origin, suggesting the capability of tumor-initiating stem cells to in vivo differentiate also in endothelial cells. In conclusion, we demonstrate that CD105⁺ cells and clones derived from renal carcinomas were enriched in tumor-initiating cells with stem characteristics.—Bussolati, B., Bruno, S., Grange, C., Ferrando, U., Camussi, G. Identification of a tumor-initiating stem cell population in human renal carcinomas.