The present invention is directed to a new bone marrow stromal stem cell (BMSSC) marker, CD18, for use in selecting a population of cells enriched in BMSSCs, from bone marrow cells, adipose cells, or peripheral blood. The invention is further directed to methods for selecting a population of cells enriched in BMSSCs based on the selective expression of CD18 on their surface, using techniques known in the art such as fluorescent assisted cell sorting, an immunomagnetic method, flow microfluorimetry, immunofluorescence, immunoperoxidase staining, radioimmunoassay and immunoaffinity chromatography. The invention is further directed to the BMSSCs isolated based on CD18 expression, and their use to treat various diseases. In one aspect, the HMSSCs are transformed with a vector having a normal gene for CD18, and the transformed BMSSCs are administered to treat bone degenerative diseases and diseases of bone involving abnormal expression of CD18 expression of CD18.