Abstract: A method for conversion of an animal into an appropriate recipient of tumor cells derived from a different species. Animals for such purpose can be immuno-incompetent animals that are doubly grafted with orthotopic tissues, in which one grafted tissue (i.e., breast) is from an organ of the same class as the tumor of origin (graft A), and the second grafted tissue (i.e., bone) is from a organ of the same class as a target organ for metastasis (graft B). These dual grafted animals can be used to model human diseases. In one implementation, human tumor cells are orthotopically seeded in graft A in order to analyze the occurrence of metastasis in graft B. Methods and compositions are described for creating a multiorgan human environment in mice, by grafting human stem cells (mesenchymal, embryonic or others) into mice, e.g., injured to enhance specific tissue engraftment. Such chimeric mice can be used to grow human tumors and to study the occurrence of metastasis.