Based on historical radiation experiments in rodents, the hematopoietic stem cell was defined by its biological properties and later by the expression of certain surface antigens (e.g., CD34), as well as the absence of lineage-specific markers (e.g., DR). Quite recently it was shown that hematopoietic reconstitution can also be achieved by CD34⁻ stem cells, which can be isolated from the bone marrow, peripheral blood and cord blood cells. CD34⁻ stem cells are considered to be predominately part of the quiescent stem cell pool of hematopoietic and mesenchymal stem cells. Due to novel techniques, CD34⁻ stem cells can be expanded on the level of a true stem cell but also directed towards their differentiation into specified tissues or organ systems. This requires the establishment of primary fibroblast-like CD34⁻ stem cells in vitro and their possible reversible and transient immortalization with optimized vector systems.