Stromal stem cells are prospectively isolated from human bone marrow then expanded into clonal populations and cultured and used, the isolation being on the basis of expression of a cell surface marker, wherein the cell surface marker binds an antibody and wherein said antibody cross reacts with a cell surface marker found on mouse stromal stem cells or rat stromal stem cells, and optionally also on a cell of at least one other mammalian species selected from mouse, rat, horse, rabbit and pig cells. Useful stromal stem cell populations are positive for SDC2.