The expectation generated by the pluripotentiality of embryonic stem (ES) cells has initiated a renaissance in stem cell biology. While ES cells can be harvested in abundance and appear to be the most versatile of cells for regenerative medicine, adult stem cells also hold promise, but the identity and subsequent isolation of these comparatively rare cells remains problematic in most tissues, perhaps with the notable exception of the bone marrow. Identifying surface molecules (markers) that would aid in stem cell isolation is thus a major goal for stem cell biologists. Moreover, the characterization of normal stem cells in specific tissues may provide a dividend for the treatment of cancer. There is a growing belief that the successful treatment of neoplastic disease will require specific targeting of the cancer stem cells, cells that may well have many of the characteristics of their normal counterparts.