A cell line isolated and established from a mammalian fetal AGM region and capable of supporting the proliferation or survival of hematopoietic stem cells and hematopoietic progenitor cells; grafts containing hematopoietic stem cells or hematopoietic progenitor cells cultured together with the above cell line and thus proliferated; gene therapy compositions containing hematopoietic stem cells or hematopoietic progenitor cells cultured together with the above cell line and thus proliferated and carrying a foreign gene transferred thereinto; a method for supporting the proliferation or survival of hematopoietic stem cells or hematopoietic progenitor cells characterized by culturing cells involving at least hematopoietic stem cells or hematopoietic progenitor cells or fractions thereof together with the above cell line; a method for transplanting hematopoietic stem cells characterized by transplanting hematopoietic stem cells or hematopoietic progenitor cells cultured together with the above cell line and thus proliferated; and a gene therapy method characterized by transferring a foreign gene into hematopoietic stem cells or hematopoietic progenitor cells cultured together with the above cell line and thus proliferated and then transplanting the cells thus carrying the gene transferred thereinto.
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