Monoclonal antibodies, or fragments thereof, are used for isolating and/or identifying at least one cell population. The cell population can include any of the following types of cells: haematopoietic stem cells, neuronal stem cells, neuronal progenitor cells, mesenchymal stem cells and mesenchymal progenitor cells. The antibodies, or fragments thereof, bind to an antigen which is the same as that bound by an antibody which is produced by the hybridoma cell lines CUB1, CUB2, CUB3 and CUB4, which were deposited in the DSMZ under the numbers DSM ACC2569, DSM ACC2566 and DSM ACC2565, on 14 Aug. 2002, and DSM ACC2551, on 12 Jul. 2002.