The invention provides methods for diagnosing tumors in mammals using a reagent that bind to LEDGF/p75 or to a nucleic acid encoding LEDGF/p75. For example, the tumor may be located in the CNS, the prostate, the skin, the bone marrow, or the gut of the mammal. Also provided are methods for diagnosing brain tumors such as medulloblastomas, meningiomas, astrocytomas, glioblastomas multiforme, and ependymomas by examining LEDGF/p75 or a nucleic acid encoding LEDGF/p75 localization. The invention also involves methods for diagnosing cancers involving cancerous epithelial cells such as colon cancer. The instant invention also provides methods for isolating stem cells from a heterogeneous population of cells, as well as methods for identifying neuroepithelial stem cells, newly differentiated neurons, and astrocytes in a subject. Also provided are methods for inducing the differentiation of neuroepithelial stem cells into astrocytes or neurons and methods for screening candidate compounds that regulate the differentiation of neuroepithelial stem cells.